It's just a pity the Optus network is absolute rubbish. ..
Heard that Optus has a patchy network but it depends where you live I guess: Had no problems on Optus (in the early 2000s) in Armidale nor when visiting Sydney. Last time in Oz I tried '3' which was on a cheap prepay deal and gave no reception probs (Sydney, Canberra, Cowra, Katoomba, etc.).