diff --git a/src/genn/backends/single_threaded_cpu/backend.cc b/src/genn/backends/single_threaded_cpu/backend.cc
index 29ac1961a6..9f00e9b0b1 100644
--- a/src/genn/backends/single_threaded_cpu/backend.cc
+++ b/src/genn/backends/single_threaded_cpu/backend.cc
@@ -1484,6 +1484,26 @@ bool Backend::isGlobalHostRNGRequired(const ModelSpecMerged &modelMerged) const
         return true;
     }
 
+    // If any custom updates require an RNG fo initialisation, return true
+    if(std::any_of(model.getCustomUpdates().cbegin(), model.getCustomUpdates().cend(),
+                   [](const ModelSpec::CustomUpdateValueType &c)
+                   {
+                       return (c.second.isInitRNGRequired());
+                   }))
+    {
+        return true;
+    }
+
+    // If any custom WU updates require an RNG fo initialisation, return true
+    if(std::any_of(model.getCustomWUUpdates().cbegin(), model.getCustomWUUpdates().cend(),
+                   [](const ModelSpec::CustomUpdateWUValueType &c)
+                   {
+                       return (c.second.isInitRNGRequired());
+                   }))
+    {
+        return true;
+    }
+
     return false;
 }
 //--------------------------------------------------------------------------
diff --git a/src/genn/genn/code_generator/backendSIMT.cc b/src/genn/genn/code_generator/backendSIMT.cc
index 9f0965aa1c..6ae18a314e 100644
--- a/src/genn/genn/code_generator/backendSIMT.cc
+++ b/src/genn/genn/code_generator/backendSIMT.cc
@@ -136,6 +136,26 @@ bool BackendSIMT::isGlobalDeviceRNGRequired(const ModelSpecMerged &modelMerged)
         return true;
     }
 
+    // If any custom updates require an RNG fo initialisation, return true
+    if(std::any_of(model.getCustomUpdates().cbegin(), model.getCustomUpdates().cend(),
+                   [](const ModelSpec::CustomUpdateValueType &c)
+                   {
+                       return (c.second.isInitRNGRequired());
+                   }))
+    {
+        return true;
+    }
+
+    // If any custom WU updates require an RNG fo initialisation, return true
+    if(std::any_of(model.getCustomWUUpdates().cbegin(), model.getCustomWUUpdates().cend(),
+                   [](const ModelSpec::CustomUpdateWUValueType &c)
+                   {
+                       return (c.second.isInitRNGRequired());
+                   }))
+    {
+        return true;
+    }
+
     return false;
 }
 //--------------------------------------------------------------------------
diff --git a/tests/unit/customUpdate.cc b/tests/unit/customUpdate.cc
index 19c64f593b..ea36020364 100644
--- a/tests/unit/customUpdate.cc
+++ b/tests/unit/customUpdate.cc
@@ -141,6 +141,94 @@ IMPLEMENT_MODEL(ReduceNeuronSharedVar);
 //--------------------------------------------------------------------------
 // Tests
 //--------------------------------------------------------------------------
+
+TEST(CustomUpdates, ConstantVarSum)
+{
+    ModelSpecInternal model;
+
+    NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
+    NeuronModels::Izhikevich::VarValues varVals(0.0, 0.0);
+    NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
+
+    Sum::VarValues sumVarValues(0.0);
+    Sum::VarReferences sumVarReferences1(createVarRef(ng, "V"), createVarRef(ng, "U"));
+ 
+    CustomUpdate *cu = model.addCustomUpdate<Sum>("Sum", "CustomUpdate",
+                                                  {}, sumVarValues, sumVarReferences1);
+    model.finalize();
+
+    CustomUpdateInternal *cuInternal = static_cast<CustomUpdateInternal*>(cu);
+    ASSERT_FALSE(cuInternal->isZeroCopyEnabled());
+    ASSERT_FALSE(cuInternal->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_FALSE(backend.isGlobalHostRNGRequired(modelSpecMerged));
+}
+
+TEST(CustomUpdates, UninitialisedVarSum)
+{
+    ModelSpecInternal model;
+
+    NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
+    NeuronModels::Izhikevich::VarValues varVals(0.0, 0.0);
+    NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
+
+    Sum::VarValues sumVarValues(uninitialisedVar());
+    Sum::VarReferences sumVarReferences1(createVarRef(ng, "V"), createVarRef(ng, "U"));
+ 
+    CustomUpdate *cu = model.addCustomUpdate<Sum>("Sum", "CustomUpdate",
+                                                  {}, sumVarValues, sumVarReferences1);
+    model.finalize();
+
+    CustomUpdateInternal *cuInternal = static_cast<CustomUpdateInternal*>(cu);
+    ASSERT_FALSE(cuInternal->isZeroCopyEnabled());
+    ASSERT_FALSE(cuInternal->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_FALSE(backend.isGlobalHostRNGRequired(modelSpecMerged));
+}
+
+TEST(CustomUpdates, RandVarSum)
+{
+    ModelSpecInternal model;
+
+    NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
+    NeuronModels::Izhikevich::VarValues varVals(0.0, 0.0);
+    NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
+
+    InitVarSnippet::Uniform::ParamValues dist(0.0, 1.0);
+    Sum::VarValues sumVarValues(initVar<InitVarSnippet::Uniform>(dist));
+    Sum::VarReferences sumVarReferences1(createVarRef(ng, "V"), createVarRef(ng, "U"));
+ 
+    CustomUpdate *cu = model.addCustomUpdate<Sum>("Sum", "CustomUpdate",
+                                                  {}, sumVarValues, sumVarReferences1);
+    model.finalize();
+
+    CustomUpdateInternal *cuInternal = static_cast<CustomUpdateInternal*>(cu);
+    ASSERT_FALSE(cuInternal->isZeroCopyEnabled());
+    ASSERT_TRUE(cuInternal->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_TRUE(backend.isGlobalHostRNGRequired(modelSpecMerged));
+}
 TEST(CustomUpdates, VarReferenceTypeChecks)
 {
     ModelSpecInternal model;
diff --git a/tests/unit/neuronGroup.cc b/tests/unit/neuronGroup.cc
index 8aeab69c55..2afc9c7f79 100644
--- a/tests/unit/neuronGroup.cc
+++ b/tests/unit/neuronGroup.cc
@@ -211,55 +211,99 @@ TEST(NeuronGroup, InvalidName)
 
 TEST(NeuronGroup, ConstantVarIzhikevich)
 {
-    ModelSpec model;
+    ModelSpecInternal model;
 
     NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
     NeuronModels::Izhikevich::VarValues varVals(0.0, 0.0);
     NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
 
+    model.finalize();
+
     ASSERT_FALSE(ng->isZeroCopyEnabled());
     ASSERT_FALSE(ng->isSimRNGRequired());
     ASSERT_FALSE(ng->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_FALSE(backend.isGlobalHostRNGRequired(modelSpecMerged));
 }
 
-TEST(NeuronGroup, UnitialisedVarIzhikevich)
+TEST(NeuronGroup, UninitialisedVarIzhikevich)
 {
-    ModelSpec model;
+    ModelSpecInternal model;
 
     NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
     NeuronModels::Izhikevich::VarValues varVals(uninitialisedVar(), uninitialisedVar());
     NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
 
+    model.finalize();
+
     ASSERT_FALSE(ng->isZeroCopyEnabled());
     ASSERT_FALSE(ng->isSimRNGRequired());
     ASSERT_FALSE(ng->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_FALSE(backend.isGlobalHostRNGRequired(modelSpecMerged));
 }
 
-TEST(NeuronGroup, UnitialisedVarRand)
+TEST(NeuronGroup, RandVarIzhikevich)
 {
-    ModelSpec model;
+    ModelSpecInternal model;
 
     InitVarSnippet::Uniform::ParamValues dist(0.0, 1.0);
     NeuronModels::Izhikevich::ParamValues paramVals(0.02, 0.2, -65.0, 8.0);
     NeuronModels::Izhikevich::VarValues varVals(0.0, initVar<InitVarSnippet::Uniform>(dist));
     NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::Izhikevich>("Neurons0", 10, paramVals, varVals);
 
+    model.finalize();
+
     ASSERT_FALSE(ng->isZeroCopyEnabled());
     ASSERT_FALSE(ng->isSimRNGRequired());
     ASSERT_TRUE(ng->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_TRUE(backend.isGlobalHostRNGRequired(modelSpecMerged));
 }
 
 TEST(NeuronGroup, Poisson)
 {
-    ModelSpec model;
+    ModelSpecInternal model;
 
     NeuronModels::PoissonNew::ParamValues paramVals(20.0);
     NeuronModels::PoissonNew::VarValues varVals(0.0);
     NeuronGroup *ng = model.addNeuronPopulation<NeuronModels::PoissonNew>("Neurons0", 10, paramVals, varVals);
 
+    model.finalize();
+
     ASSERT_FALSE(ng->isZeroCopyEnabled());
     ASSERT_TRUE(ng->isSimRNGRequired());
     ASSERT_FALSE(ng->isInitRNGRequired());
+
+    // Create a backend
+    CodeGenerator::SingleThreadedCPU::Preferences preferences;
+    CodeGenerator::SingleThreadedCPU::Backend backend(model.getPrecision(), preferences);
+
+    // Merge model
+    CodeGenerator::ModelSpecMerged modelSpecMerged(model, backend);
+
+    ASSERT_TRUE(backend.isGlobalHostRNGRequired(modelSpecMerged));
 }
 
 TEST(NeuronGroup, FuseWUMPrePost)