In the past 24 hours Timaru has been drenched with three times the amount of rain it's received in the last month.

It may only be 29mm of rain, but it's a welcome downpour for the town and farmers, after Timaru only saw 8mm of rain fall in the past 30 days.

Rain is expected to fall in the area until Thursday afternoon, says MetService meteorologist Amanda De Monte.

"Considering how dry it's been for the past month, this rainfall is pretty significant," she said.

Meanwhile, rain has brought relief to the Garden City which went 46 days straight without a drop, breaking a record from 1954.

Thunderstorms are starting to develop at the Canterbury foothills, just west of Christchurch and will stick around until Thursday, according to MetService.

In the North Island, severe thunderstorms had also developed near Gisborne and Wairoa on Wednesday afternoon, the MetService says.

The severe weather and very heavy rain is expected to have hit Tiniroto, Ruakituri, Lake Waikaremoana, Ngatapa and surrounding districts by 4.30pm.

The affected areas are close to the Mahia Peninsula where New Zealand-based company Rocket Lab on Thursday afternoon hopes to launch its Electron rocket into space.

The launch is part of a three flight test programme as the company prepares to become a commercial operation.