NIWA warns New Zealand is set to scorch on Tuesday with a hottest temperature of nearly 35C.
A high is bringing fine and settled weather to much of the country, but will also push up the mercury.
"Along with the sunshine, will be the heat. How high will it go today?" NIWA asks on Twitter.
"Our high-resolution model is showing near 35C in eastern Canterbury, north of Christchurch."
WeatherWatch says that inland parts of every South Island region will have daytime highs over 8C above average and Fire and Emergency warns all of Canterbury will be moving to a restricted fire season from midnight Wednesday.
"There are a few 30C days forecast for Canterbury later this week. This heat, combined with the dry and windy conditions, increase the fire danger and risk of a wildfire," Region Manager Paul Henderson says.
"There have already been three significant fires across Canterbury this fire season - Lake Pukaki, Lake Ōhau and last weekend's fire in Hillsborough on the Port Hills.
"People need to take the fire risk seriously."
Henderson says a permit may be required to light a fire outside and he asks the public not to let off fireworks or create sparks during the heat.
"In these conditions a firework, or spark from machinery, could easily start a wildfire which would get quickly out of control."
With a run of hot temperatures over the next seven to ten days, NIWA says New Zealand faces a 'warm blob' in the ocean.
"This will likely lead to a dramatic warm-up in our coastal water temperatures leading up to Christmas," it says.
"It will also become increasingly more humid."
