I don’t want to make assumptions about how you use the services in MyGov, but the experience of my family and friends has been that, if you require any service which is at all out-of-the-ordinary or needs a pair of human eyes looking at it (whether or not it should - there are plenty of broken processes at Centrelink, some might say that’s by design) then you need to make a phone call, because the function to perform self-service is simply not there. I have also had fairly good experiences with MyGov - but my interactions with social services are fairly boring compared to the needs of many people.

Edit: the problem here of course is that if you make a phone call you can expect to wait in a queue for many hours.