If you're close to Bunnings, buy some "Tomcat" Rat Baits. They are in blocks and pretty cheap.
The next problem is finding a way to get the baits in the roof. If the rats can get in there I'm sure there is a way.
Alternatively, if you have a spare $120-$160 you can call someone out and get reimbursed from the realestate.

Hope they go away ASAP!