Most of the hire places have baby seats but often they are a bit dodgy. Some of the people who work there have no clue about the correct seat for age and some of the companies provide the most basic seat they can get away with. Make sure you check that they have installed it properly if you use theirs.
If you can it's beter to take your own. Most airlines will give you a bit of leeway on your luggage allowance if you're carting around baby stuff. Just ring up first or read the T&C's to be sure. Virgin Blue counts each piece of baby stuff (seat, porta cot ect) as 5kg regardless of its actual weight.