The monitoring has ensured that the issue has now been fully resolved.

We have identified an issue on API submissions to the gateway that would have affected any API submissions between 15:53 and 16:00 AEST. Submissions during this period may have been dropped, and will need to be resent if your API request received a 5XX error.
We have resolved this issue and are currently monitoring the status.
An update will be provided at 17:00 AEST.