Weather: Flooding fears as 'deluge' bears down on South Island

Two fronts will bring rain this weekend as a "deluge" bears down on the South Island with warnings of possible slips and flooding.

But MetService says unfortunately the rain won't hit the drought-stricken areas in the North Island that need it.

"The first cold front moves up the South Island this afternoon and evening [Thursday], while overnight tonight the second cold front moves north," MetService says.

WeatherWatch says Friday's "deluge" could bring 200mm to the West Coast within 24 hours.

"This could cause flooding, slips and some road closures," the forecaster said in a statement.

Heavy rain warnings are in place for the Buller Ranges, and Westland south of Otira.

It's a stark contrast on the East Coast of the South Island - with Christchurch and Dunedin expected to reach 30C and 25C respectively on Friday.

"As the combined cold front moves north it begins to lose its intensity over the centre of the North Island, before fading away about Rotorua and Hawke's Bay," MetService says.

"Meanwhile, a ridge of high pressure builds across the South Island during Saturday and Sunday, eventually moving over the North Island on Monday, bringing settled weather."