TL;DR: Melbourne spent over $1.5B developing their very own tap to pay public transport card from scratch, which was and remains slow, glitchy and is already outdated (eg doesn't support payment with contactless credit/debit cards). Meanwhile, Sydney deployed an off the shelf system from Cubic at a fraction of the cost and the rollout was about as smooth as it gets.
https://en.wikipedia.org/wiki/Myki
TL;DR: Melbourne spent over $1.5B developing their very own tap to pay public transport card from scratch, which was and remains slow, glitchy and is already outdated (eg doesn't support payment with contactless credit/debit cards). Meanwhile, Sydney deployed an off the shelf system from Cubic at a fraction of the cost and the rollout was about as smooth as it gets.