A slow-moving trough lying over Coromandel Peninsula early on Monday morning may bring a period of heavy rain and possible thunderstorms during the early morning, says a MetService spokesperson.

"There is low confidence of rainfall amounts reaching warning criteria.

"A strengthening northwest flow should then spread onto the South Island on Tuesday, bringing rain to western areas. There is low confidence rainfall amounts will reach warning criteria in Fiordland on Tuesday."

MetService says fronts embedded within the strong northwest flow should bring periods of heavy rain to the western South Island on Wednesday and Thursday, as well as gale or severe gale northwesterlies to parts of central and southern New Zealand.

"During this time, there is moderate confidence of warnable amounts of rain for Fiordland and Westland south of Otira, with spillover into the headwaters of the Otago and Canterbury Lakes and Rivers also likely.

"For the remainder of Westland and Buller, there is low confidence a heavy rain warning will be required for Wednesday and/or Thursday. Additionally, there is a low risk of rainfall amounts reaching warning criteria about the Tararua Range and Mt Taranaki on Thursday."

The strong northwest flow also means there is moderate confidence that northwest winds may reach severe gale at times on Wednesday and/or Thursday about exposed parts of Fiordland, Stewart Island, Southland, inland Otago, Canterbury, the Marlborough Sounds, Wellington and Wairarapa, says MetService.

This MetService Severe Weather Outlook is valid from Monday March 6 to Friday March 10.