diff --git a/lib/workers/repository/update/branch/reuse.spec.ts b/lib/workers/repository/update/branch/reuse.spec.ts
index 8f47e9782763ed..d5113d962ab7a5 100644
--- a/lib/workers/repository/update/branch/reuse.spec.ts
+++ b/lib/workers/repository/update/branch/reuse.spec.ts
@@ -262,5 +262,15 @@ describe('workers/repository/update/branch/reuse', () => {
       expect(config.rebaseWhen).toBe('auto');
       expect(result.rebaseWhen).toBe('conflicted');
     });
+
+    it('converts rebaseWhen=auto to behind-base-branch if automerge is true AND branch is new', async () => {
+      config.rebaseWhen = 'auto';
+      config.automerge = true;
+      scm.branchExists.mockResolvedValueOnce(false);
+      scm.isBranchBehindBase.mockResolvedValueOnce(false);
+      const result = await shouldReuseExistingBranch(config);
+      expect(config.rebaseWhen).toBe('auto');
+      expect(result.rebaseWhen).toBe('behind-base-branch');
+    });
   });
 });
diff --git a/lib/workers/repository/update/branch/reuse.ts b/lib/workers/repository/update/branch/reuse.ts
index 9a915480b9008b..7b052eeebf1ad9 100644
--- a/lib/workers/repository/update/branch/reuse.ts
+++ b/lib/workers/repository/update/branch/reuse.ts
@@ -31,14 +31,16 @@ export async function shouldReuseExistingBranch(
 ): Promise<BranchConfig> {
   const { baseBranch, branchName } = config;
   const result: BranchConfig = { ...config, reuseExistingBranch: false };
+
+  const keepUpdated = await shouldKeepUpdated(result, baseBranch, branchName);
+  await determineRebaseWhenValue(result, keepUpdated);
+
   // Check if branch exists
   if (!(await scm.branchExists(branchName))) {
     logger.debug(`Branch needs creating`);
     return result;
   }
   logger.debug(`Branch already exists`);
-  const keepUpdated = await shouldKeepUpdated(result, baseBranch, branchName);
-  await determineRebaseWhenValue(result, keepUpdated);
 
   if (result.rebaseWhen === 'behind-base-branch' || keepUpdated) {
     if (await scm.isBranchBehindBase(branchName, baseBranch)) {