I don't think that 3 roams onto Telstra 3G, only Telstra 2G, otherwise it would be the cheap way to take advantage of Telstra's coverage without their impressive bills. Tel 2G cover ain't that flash.
'3' actually do roam on the Telstra 3G and 2G networks Telstra invested in the '3' network to negate the need to build their own and instead built a 3G HSDPA network which they call the Next G network.
I can't stand Telstra, the customer service is crap, at takes you an hour to speak to the right department by which stage you need to send your phone in for repairs after kicking it into a wall and if it were not for the coverage I would be long gone.
My best experience as a customer was with vodafone and if they built a 3G HSDPA network i'd be there in a heartbeat.
Solution for you mate is to have a cheap Next G prepaid phone that you take flying with you and use your other phone in the city. I think they stopped making dual sim readers after the 5110/3310/8210 style phones which are the only ones I have ever seen them for.