Weather: Monster storm brewing for Fiordland

South Islanders are battening down the hatches as a storm prepares to unleash upon them.

A front is forecast to move onto the lower South Island on Monday until Wednesday, and MetService is warning of severe gales for exposed places in Southland.

Heavy rain will also strike the west of the lower South Island and up to a third of a metre of rain could fall during this period.

"A slow-moving front brings rain and strong northwesterlies to Fiordland, southern Westland and coastal Southland. Elsewhere cloudy periods and a few showers are the order of the day," MetService says.

"There is a Severe Weather Warning in place for heavy rain for Fiordland and southern Westland, where 300-350mm may accumulate by Wednesday morning, and a Watch for severe gales for exposed places in Southland."

NIWA warns our sea surface temperatures around New Zealand remain above average, making the storms worse.

"The seas might also contribute heat, moisture, and energy to storm systems as they approach New Zealand from the west and north during late March and at times through April," it says.

"Remember, tropical cyclone season doesn't end until 30 April - still several weeks to go!"