When ATO and Medicare don't have any means to send them a message.
ATO put me onto some sort of a weird plan where I need to pay quarterly instead of doing all taxes all in one go at the end of the year. Got a letter in MyGov "Welcome to this plan, happy days!" with no explanation for why. And there's absolutely zero way to reply and ask for more information.
I'd also need to change a detail in Medicare but there's no info how exactly to request for it. And no way to send them a secure message asking about it.
Both organisations rely on their call centres. Conversely, I try to desperately avoid call centres as most of them are hopeless and you get a run-around or a "system says no". I'd prefer a message, an internal 1st tier team directs it to the right place, a competent person takes a look and replies or calls with the right info. Case closed in minimum time & effort.
At least Medicare has a service centre a few suburbs up the road, open between 11:58 - 12:01 if the barometric pressure is 1030 and it's a day of super moon. I might be pity enough to put a podcast on and go & queue up there...