Weather: Sweltering temperatures climb as heatwave sweeps NZ

Sweltering heat has continued to bake New Zealand on Saturday, with Wanaka reaching 35degC.

There's a warning for those struggling to cope, with forecasters warning it will get hotter before it gets cooler.

As a stoker on Lake Wakitipu's TSS Earnslaw steamship, Cameron Mackenzie is used to extreme heat.

"It can get well over 50degC, but luckily we got the fans over us," he says.

Mr Mackenzie was shovelling away on Saturday with only the sweet relief of a couple fans, as outside in Queenstown, locals and tourists alike were met with a comparatively cooler 30degC.

"She's been a scorcher down here in Queenstown for the past month or so - probably one of the hottest summer's we've had," says local Jack Pelter.

Queenstown, Wanaka and the wider Otago region are bearing the brunt of a high that has brought extreme heat to much of the country.

"I think we're pretty careful not to complain about it because winter will be on our backs soon," says one resident.

MetService says the heatwave is being driven largely by higher-than-usual sea temperatures.

"It's the thing that's pushed it over the brink," says meteorologist Erick Brenstrum. "The sea temperatures are around 4decD or 5degC above average, which is something I've never seen before.

"That's a consequence of the anti-cyclone we had in November."

The hot weather is also impacting the country's rail network, with some lines in the lower South Island and Wellington and Auckland metro areas forced to close or have speed restrictions put in place. KiwiRail says this is due to the increased risk of buckling.

In Christchurch, organisers of the Great Kiwi Beer Festival couldn't have timed the event any better, with 12,000 punters gathering in Hagley Park for a day of hydration as temperatures crept into the high 20s.

"It's warm and the drinks are cold - it's good," says one attendee.

MetService is forecasting Sunday and Monday to have similar weather to Saturday before it gets even hotter, with 40degC possible in the South next week.

"That's a possibility in parts of central Otago on Tuesday," says Mr Brenstrum.

But on Wednesday one extreme could be replaced by another, with Mr Brenstrum saying the tail of a tropical cyclone is set to hit New Zealand, bringing with it heavy rain to the north and west of both islands.