Another update. i managed to get it working after updating my APN (access point names) to the latest values on this website for Optus:
This website also has other Australian APN’s listed - pretty useful
Name OPTUSMMS 3
Multimedia message proxy 126.96.36.199
Multimedia message port 8070
Authentication type PAP ( i used none to get it working)
APN type mms
APN protocol IPv4/IPv6
APN roaming protocol IPv4
after updating, it still didnt work. i restarted my phone, then
turned mobile data off.
sent an MMS
turned mobile data on
and it started working normally
after this, i could send and receive MMS as you would normally expect to be able to.