Another factor in all this is resource consent - you can't just buy land somewhere and plonk a stadium on it. Newtown is all residential and there's no way it could happen there. Petone might work but you still have to consider noise levels and light pollution from stadium light towers, not to mention parking and other infrastructure concerns like impact on traffc flows etc. Not only is the current stadium close to a public transport hub, but its also in the middle of a commercial/industrial area by major arterial roads. If they could build another smaller stadium somewhere near the current one it could work, but I can't see one anywhere else ever getting off the ground.
Anyway I think this is just a bluff by Welnix to get the council behind them a bit more.