Heavy, thundery downpours are set to dominate the weather from Sunday, according to MetService.
Media and Communications Meteorologist Daniel Corbett said the weather system will have two parts.
"First, milder northwesterlies are expected to bring persistent, and sometimes heavy, rain to western parts of both islands from Sunday through to about Tuesday," Corbett said.
"What's important about this upcoming rain event is that it's likely to go on for several days in some places and be thundery at times."
The northwest winds are expected to bring much warmer conditions to eastern areas, with daytime highs in the high teens on Sunday and Monday. But the warm temperatures will then make way for a cold southwesterly.
MetService said rainfalls in western parts of New Zealand from Sunday through to Tuesday could turn out to be quite significant.