I had our Tier 2 take a look at the logs again and we don't see any major delays happening on our end. They had the following comments:
"There are quite a few deliveries with no delay on our side. The logs show 13 seconds from accepted message to delivery to extmail.optusnet.com.au. This could be happening before the mailmap (before us) or at extmail.optusnet.com.au. Unfortunately it's impossible to say, but it's not being held on our system."