Niwa meteorologist Chris Brandolino said significantly warmer temperatures were expected across the South Island in the second half of the month.
Temperatures were expected to reach near or above 30C in some parts.
"Maximum temperatures could be towards the end of the week and over the next week we're looking at 10 to 15C above average for the South Island," he said.
"This time next week that's when I think the conversation really starts to heat up."
He described it as unusual warmth that had meteorologists concerned given the lack of rain.
At this time of year successive long, hot days of dry weather threatened to have a severe impact on the agricultural sector, he said.
While parts of coastal South Island had enjoyed good rainfall in recent weeks other inland areas were potentially more vulnerable.
In Niwa's latest three-monthly outlook the West Coast, inland Otago and Southland were expected to have temperatures above average and below normal rainfall.