For the section from the cat to the muffler should easily be done for $150 or less. Depending on whether it's a stock system, the factory piping is very cheap to replace. Can't help you out with Sydney exhaust shops as I'm in Brisbane.

On the topic of dodgy exhausts, I dismantled mine the other day and discovered some knob has hollowed the cat and put a straight through pipe. No wonder the exhaust was so pungent. $190 supplied + fitted for a replacement for reference.