Go Telstra - as much as I hate their ripoff attitude, they have by far the best network. By a lot.
Look at buying the phone outright (about $850) from Apple and buying a prepaid sim.
People incorrectly mistake prepaid as being for teenage school kids and unsuitable for business. Wrong!
Since prepaid is marketed at kids, it offers good value with a lot of calls, txt and data (use your credit balance to buy data packs).
$30 = 100min talk, 200txt & 450mb
$40 = 200min talk, 400txt & 600mb
$50 = 200min talk, 400txt & 2gb