When you’re leaving from Australia to Bali there are a couple of airlines with direct flights. Closest Australian city to Bali is Darwin with flights operated by Airnorth, Jetstar/Qantas and Garuda Indonesia.
This last airline is not even allowed to land in Europe anymore so it’s not recommended unless you’re into swimming to Bali.
Anyway these flights to Bali take around 2 hours and 30 minutes.
From Perth there are enough direct flights to Bali with Virgin Blue, Jetstar, Ozjet, Qantas and Garuda Indonesia. Flight time is more or less 3 hours and 35 minutes.
From Melbourne the quickest and only direct flight to Bali is with Garuda Indonesia in 6 hours or with one stop with Jetstar Airways and Qantas in 10 hours or Virgin Blue in 10 hours and 15 minutes.
From Sydney there are also direct flights operated by Jetstar/Qantas and the fastest flying time is 6 h 25 minutes.