Southlanders are in for some of the warmest days of the spring, with MetService forecasting a high of 24C for both Invercargill and Gore on Friday before the weather turns cooler for Saturday's election.

The forecaster said on Thursday that a plume of warm, humid air from the tropics would push temperatures above 20C across much of the country over the next few days. Hastings could reach 30C on Friday, almost 10 degrees above its November average, and nights would stay unusually mild because the moisture-laden air slows overnight cooling.

In Invercargill, Thursday's early fog was expected to clear to a 21C afternoon before rain arrives in the evening on a developing southeasterly. Friday should start with low cloud and a few showers before fine spells and a 24C high, according to MetService's city forecast. Queenstown is forecast to reach 22C on both Friday and Saturday.

The change comes on election day. MetService expects Invercargill to be partly cloudy with southwesterlies and a top of 17C on Saturday, with a few showers developing in the evening, while Gore should see a 18C high and a mix of cloud and sunshine. Sunday looks cooler again, with highs of 16C and afternoon showers across the region.

MetService meteorologist Alanna Burrows said thunderstorms were a risk for inland parts of the North Island on Thursday, and that the risk would build on Friday and again on Saturday, with parts of the South Island also in the frame heading into the weekend. She encouraged anyone heading out to vote to keep an eye on the rain radar and the latest forecast.

As of midday Thursday there were no severe weather warnings or watches in force for Southland or Fiordland on MetService's alerts feed.

For Guy Fawkes night, Burrows said conditions should not cause much trouble in the main centres, with light northerlies, and that finding a gap in the cloud would be the main challenge. Fire and Emergency has asked people to check the fire danger at checkitsalright.nz before lighting any fires outdoors, with pockets of high and very high fire risk around the country.

The warm spell follows a mild October. Earth Sciences New Zealand's monthly climate summary, released on Thursday, said most of the South Island ran more than 1.2 degrees above average for the month. Rainfall across much of Southland and around Dunedin was close to normal, while northern and central Otago and Milford Sound were wetter than usual. Milford Sound recorded the country's biggest single-day total, with 251mm on 26 October. Southland soils finished the month with near-normal moisture levels.

Polling booths open at 9am on Saturday and close at 7pm. Advance voting continues at voting places across Invercargill and Southland until Friday evening.