'Blocking' high causes Nelson deluge

The prolonged torrential rain that saturated the upper part of the South Island is the result of a range of factors but the main cause is a blocking high pressure system sitting to the east of the country around the dateline.

MetService's Daniel Corbett said the high had been like a big boulder in a stream blocking the flow.

A "pipeline" of rain had developed as subtropical moisture barrelled in from the north Tasman Sea.

Nelson was protected from the south by mountains in a kind of U-shape, "but if you get wind from the north like we've had that can bring the moisture in", Corbett said.

The band of intense rain was moving away from Nelson around dawn today, and should clear away from the Richmond Ranges to the east by about lunchtime.

It was moving across the North Island, with up to 150mm possible in places from Taranaki up to Northland and across to the Bay of Plenty.

Civil Defence said the most significant falls of rain in the upper South Island had been in lowland areas of Takaka and Richmond, and in the hills behind Nelson.

The 510mm measured at the Kotinga gauge at Takaka township in the 48 hours to 3am was the "most impressive" rainfall total. That included 423mm in 24 hours, which was well in excess of a 1-in-100 year rainfall event.

In the 35 years of readings at that site, the previous 24-hour record was 256mm.

On the Waimea plains 200mm of rain fell in 24 hours, also greater than a 1-in-100 year rainfall, Civil Defence said.

The lowland falls combined with even larger rainfall totals in the Waimea and Maitai headwaters, where storm totals reached in excess of 400mm.

"These are substantial amounts of rain to fall on lowland areas, particularly urban areas, and there was considerable flooding from small streams, while more moderate floods occurred in the larger rivers," Civil Defence said.