the bolts will be standard size.. at a guess i'd say you're looking at M8.. go to bunnings/mitre 10 and just buy a cheap M8 bolt.. and test it.. will only cost 20 cents or something.. and if you really wanna be tight.. just exchange for something else until you get it right..
Or heaven forbid if you actually find someone in there that knows what they are talking about, you could take the car with you, they'll come out the front and should be able to tell, or even try some for you.. Or hopefully they would even have a thread gauge