diff --git a/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/Messages/UserMergedMessage.cs b/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/Messages/UserMergedMessage.cs
index c09880a5c..5c5887aa9 100644
--- a/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/Messages/UserMergedMessage.cs
+++ b/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/Messages/UserMergedMessage.cs
@@ -5,5 +5,5 @@ public record UserMergedMessage : INotificationMessage
     public const string MessageTypeName = "UserMerged";
 
     public required Guid MergedUserId { get; init; }
-    public required User MasterUser { get; init; }
+    public required Guid MasterUserId { get; init; }
 }
diff --git a/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/PublishNotificationsEventObserver.cs b/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/PublishNotificationsEventObserver.cs
index 8da08ba93..00a9d7e13 100644
--- a/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/PublishNotificationsEventObserver.cs
+++ b/dotnet-authserver/src/TeacherIdentity.AuthServer/Notifications/PublishNotificationsEventObserver.cs
@@ -74,8 +74,8 @@ private static IEnumerable<NotificationEnvelope> GetNotificationsForEvent(EventB
                 NotificationId = Guid.NewGuid(),
                 Message = new UserMergedMessage()
                 {
-                    MasterUser = userMerged.User,
-                    MergedUserId = userMerged.MergedWithUserId
+                    MasterUserId = userMerged.MergedWithUserId,
+                    MergedUserId = userMerged.User.UserId
                 },
                 MessageType = UserMergedMessage.MessageTypeName,
                 TimeUtc = userMerged.CreatedUtc
diff --git a/dotnet-authserver/tests/TeacherIdentity.AuthServer.Tests/WebHooks/WebHooksEndToEndTests.cs b/dotnet-authserver/tests/TeacherIdentity.AuthServer.Tests/WebHooks/WebHooksEndToEndTests.cs
index 53dbc1b08..131f31133 100644
--- a/dotnet-authserver/tests/TeacherIdentity.AuthServer.Tests/WebHooks/WebHooksEndToEndTests.cs
+++ b/dotnet-authserver/tests/TeacherIdentity.AuthServer.Tests/WebHooks/WebHooksEndToEndTests.cs
@@ -381,7 +381,7 @@ public async Task UserMerged_WhenPublished_IsOnlySentToWebHookEndpointsWhichSubs
             NotificationId = Guid.NewGuid(),
             Message = new UserMergedMessage()
             {
-                MasterUser = user,
+                MasterUserId = user.UserId,
                 MergedUserId = mergedUserId
             },
             MessageType = UserMergedMessage.MessageTypeName,
@@ -408,19 +408,7 @@ public async Task UserMerged_WhenPublished_IsOnlySentToWebHookEndpointsWhichSubs
                     notificationId = userCreatedNotification.NotificationId,
                     message = new
                     {
-                        masterUser = new
-                        {
-                            userId = user.UserId,
-                            dateOfBirth = user.DateOfBirth,
-                            emailAddress = user.EmailAddress,
-                            firstName = user.FirstName,
-                            middleName = user.MiddleName,
-                            lastName = user.LastName,
-                            preferredName = user.PreferredName,
-                            mobileNumber = user.MobileNumber,
-                            trn = user.Trn,
-                            trnLookupStatus = user.TrnLookupStatus.ToString()
-                        },
+                        masterUserId = user.UserId,
                         mergedUserId = mergedUserId
                     },
                     messageType = UserMergedMessage.MessageTypeName,