FYI under Australian law you need minimum 1.5mm tread depth to be legal - you should find out what your local law says.
From my experience you get roughly 1.5 - 2 front tyres for each rear one so I'd be looking at replacing the front now - probably with a metzler or avon (I've recently put a michelin on the front and found it cheaper and just as good as the metzler) then do the rear when it gets down to about 2mm tread depth. This gets value out of your tyres and keeps you on the safe side of legal. It also means you're not running in two slippery new tyres at the same time.
