This site here backs up what you said, and as you mentioned its really not close. My personal experience was based on short term rentals (<1 year).

https://www.bedssi.com/rental-price-guide-for-australian-cit...

I did find that Sydney had more availability and options (at least during the market period I was searching). Lower Supply and High Demand should drive up Melbourne's price, but as you said, real estate in Sydneys CBD is just more valuable than Melbourne.