Growth rates are quite slow, a 10 kg adult is probably 20 years old, and a fish at the maximum size of 1.3 m long and 20 kg is probably 50 years old. Sexual maturity is reached at about 30 cm long and a small percentage of the males will actually turn into females at puberty. Anglers are advised not to take immature fish, so as not to reduce breeding stock. The legal size in Australia varies by state, from 25 cm in Queensland to 41 cm in Western Australia.
This took a little bit of digging up but i think it will answer your question.
The size limits are a little out of date.
Graeme