im a messenger in adelaide, aus. i know a few in Melbs and like anywhere else it's competitive to get a job pushing the envelopes. there's so few positions it's all about who you know. definitely comes with an elitist crowd. but start with researching the companies in Melb and hit them up, try meeting other riders. theres a subreddit for messengers worth checking out.
otherwise foodie apps are a big thing down there and they hiring anyone. Uber eats and Deliveroo. might wanna try starting with them until you get a lead on paper