Magic Bus have fares that look pretty good – but only for the next 6 days. They go up on Novemeber 1st as the new high season approaches.
Personally I am going to book the North and South trip – at $459 (~£160) to do the highlights of both islands it looks top value. I will probably then fly back to Auckland – fares are as low as $90 (~£35), which is cheap considering returning by bus would incur another ferry charge.
So my plan now is to kick back, relax and treat the first month as a holiday, see the sights and then decide on where to go back to, whether to buy a car etc etc.