Hi John, good question!  The main difference between a deep low in the Southern Ocean and a deep low in the tropics is the centre.  Tropical lows are usually very tightly wrapped up – and the damaging portion is severe around that centre.  With these lows the air pressure can be similar but the worst of the winds tend to be spread over a much greater area.  If it had made a direct hit it would’ve meant a lot more wind – damaging gales south of the lower North Island – and perhaps more importantly – the southerly blast would’ve extended up into the sub-tropics, meaning snow would’ve fallen lower, further north.  Luckily most of these storms weaken if they move too far north towards us – but every one of them is worth monitoring closely as they can be quite moody!  So because the winds are spread out further than Katrina, we wouldn’t see the same storm surge or destructive winds. Nor would we see as much rain, as tropical lows create torrential downoours, but these Southern Ocean lows having drier air – so we get more showers.

Hope that helps!
Cheers
Phil