Sorry, I was in a rush to catch my train to work, so didn't have time to say much other than provide an identification. Juveniles are fairly common in your neck of the woods in the summer through to early winter, usually in rock pools and estuaries. They can also get into freshwater. (I've even seen them in storm drains in Newcastle.) They are often kept as aquarium fish, usually in brackish water tanks, but are a bit too aggressive and large (to around 25 cm or so) for my money.
Tony