Torrential rain threatens to come for an extended period in South Island with gale-force winds gusting to 160km per hour, yet North Island, in need of rain, might get none.

Much of this is due to the precipitous deepening low coming in from the west, and it is attributed to a process termed, explosive cyclogenesis, which speaks primarily to how rapidly it is deepening.Meteorologists are also calling such an event a bomb low.

One MetService meteorologist in particular, Lisa Murray, said, "It usually happens in winter.To get this really quick deepening, you need a good contrast of warm air and cold air.We don't often get it in summer."

The low dragged very cold air due north from the south, and this led to precipitous drops in temperatures covering many areas but hitting the south much harder than the north.

MetService warned of a "prolonged period" during which rain would be heavy and accumulate on the earth as much as 400mm between Tuesday evening and Thursday morning in the Westland ranges.

Murray explained further, "Imagine that just suddenly coming on the roads.There's still plenty of tourists around, people are still on holiday." This made it possible for worse inundation, surface flooding and landslides to occur, and the powerful winds will slant the rain to an almost horizontal extent sometimes.

The Canterbury lakes' and rivers' headwaters are also predicted to overflow under 300mm of rain about the main divide during the 18 hours of torrential downpour.