My experience is that you do not need a work permit if you are playing concert performances which are not held in premises that serve alcohol. This seems to apply to a lot of festival tents over there. They will want to see your contract at the border and, if you're gig is an event they know about or can verify easily, you should be OK. If it's a more obscure event, they might be suspicious. Also, if you're giving workshops it counts as teaching and it does require a work permit.

Last time I went in I got caught out on the workshop thing, but even so they issued a visa then and there at the border, so I wasn't held up too much - although I was $250 poorer.