It's not only been a staggeringly wet year but the wettest week in September is about to strike.

Niwa says some New Zealand cities have already seen 12 months worth of rain fall within the space of eight months. Christchurch, Tauranga and Auckland have already exceeded their normal annual rainfall totals while Dunedin, Wellington and Hamilton are not far off, matching their annual totals with months to spare.

And it's not about to let up with a heavy rain set to drench western regions of both islands for the next seven days.

Gales are also set to slam central New Zealand and snow will fall down to 800m across parts of the South Island in coming days.

Advertisement

"At this stage we'll see bursts of rain and strong winds for the next week or so," said MetService forecaster Tom Adams.

"We'll see pretty significant rainfall in the west for this one."

Heavy rain warnings are in force for Taranaki, Tararua Ranges, Westland and Fiordland.

MetService is warning the rain, that will mainly affect western regions of both islands, will last until the weekend and be so heavy in some parts it could lead to flooding, cause slips and make driving hazardous.

Strong winds will lash Wairarapa, Wellington, Marlborough Sounds and the Canterbury High Country from early tomorrow morning.

There is also a high chance of thunderstorms rumbling over western areas throughout today.

Snow is expected to start falling in Fiordland early tomorrow with up to 5cm expected to accumulate on Milford Rd by lunchtime and further flurries near the Homer Tunnel falling across the rest of the day.

Adams said the rain and wind would begin to ease on Thursday before a second round of rough weather slammed into the country later in the week.