The weather is due to a series of low-pressure weather systems coming from the northwest. Ahead of them are persistent north to northwesterly winds helping to drag warm moist air across the country.
The South Island will be less affected with just slightly above average overnight temperatures expected.
But daytime temperatures could reach near 30C in some places.
"There's a couple of things that make it a little more complicated, especially the Canterbury Plains and North Otago, their daytime temperatures will be reasonably warm.
"They're likely to get quite strong northwesterlies and as they come over the alps they descend to the plains and warm up."
Temperatures are expected to reach 27C in Ashburton and Timaru as well as 26C in Christchurch today.
McInnes said the warm temperatures could spell trouble for some people, especially in Hawke's Bay and Gisborne, and people should take care.
"Warmer temperatures are not that common and we're used to thinking of them as being great and nice but there can be other effects with these extended periods of heat.
"It's a good reminder to stay cool and stay hydrated. Keep alert for things as high temperatures can affect people."
In spite of the heat, a number of severe weather warnings are in place around the country.
An orange heavy rain warning is in place for the ranges of Westland south of Otira and a heavy rain watch is in place for Mt Taranaki, the Tasman and Buller ranges and Fiordland north of Doubtful Sound.
An orange strong wind warning is in place for Wairarapa and Wellington as well as the Canterbury High Country.
Strong wind watches are in place for Queenstown Lakes and Central Otago, Fiordland, Taihape, the Hawke's Bay ranges and Marlborough.
