Mt Cook and QF guys have been complaning about the high number of TCAS alerts receieved west of the airport when joining downwind at 2000ft(with good reason).
This has been mostly caused by light aircraft popping up 100ft-200ft while tracking out to the west.
So the ATC solution is to increase to 1000ft sep and only descend aircraft to 2500ft(3500ft over city) until about mid-base/long finals. Seems to be working ok, except it's a lot harder to get down from 2500ft with a late descent clearance, than it is from 2000ft! I've heard a few have been pushing through the centreline to try and get down and a couple of go-arounds have already occured because of it.
Early days though.