Campers and holidaymakers across New Zealand can mostly relax with warm, fine weather today and for the rest of the week, but some areas will need to keep umbrellas and decks of cards close to hand, with pockets of showers on the way.

While most of the country relaxes in balmy temperatures today and recovers from last night's revelry, thunderstorms are expected for inland parts of the central North Island this afternoon and evening.

As the afternoon heats up, Rotorua, Taupo, eastern Taihape and inland parts of Gisborne and Hawkes Bay, especially near the ranges, could see thunderstorms today and into this evening, with "localised downpours" of 25-40mm per hour.

Forecasters have warned that the heavy rain could cause surface flooding or flash flooding and have warned of slips and hazardous driving conditions.

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"One place could get a very heavy downfall, and somewhere not far down the road could not get very much," MetService meteorologist Claire Flynn said.

The other dodgy area of New Zealand to be in today is Fiordland and southern Westland where a front is dumping heavy rain. The region could also get thunderstorms, while scattered rain is predicted for areas of Southland and Otago.

Today's weather isn't too dissimilar to the opening day of 2018 – which saw a warm cloudy day for Auckland, early 20Cs in Wellington and Christchurch, while Dunedin basked in a stunner with a high of 23.9C on January 1 last year.

For the rest of this week, areas of the North Island will see spells of drizzle or showers, but the temperatures should hold steady and any bad weather likely to move through quickly.

It's a similar story for the South Island this week, with any showers soon clearing, and holidaymakers unhindered to enjoy the beach, lakes, hills, and al fresco dining.

The mercury is expected to soar in Christchurch tomorrow, with the Garden City likely sweating in 31C.

Today's temperatures:

• 29C for Te Kuiti, Taumarunui, and Masterton

• 24C Auckland

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• 27C Hamilton

• 24C Wellington

• 24C Christchurch

• 23C Dunedin