Some regions are experiencing below-average rainfall, particularly in the lower North Island and eastern South Island.
Christchurch has received only 5.2mm of rain so far this month, compared to 76mm last year in May. Its driest May on record was in 2012, with just 11.2mm.
The dry spell is set to continue into the weekend for Christchurch with possible showers on Monday and Tuesday.
High pressure is expected to keep conditions fairly calm through Friday and much of Saturday, MetService said.
By Sunday, rain is expected to develop in parts of the South Island and gradually move north, with showers becoming more widespread into early next week.
Auckland is expected to be hit by heavy rain and strong northeasterlies on Sunday.
While the Wellington region is forecast to be hit with heavy rain on Monday.
MetService said the South Island would be "mainly fine" over the weekend, with heavy rain in the west on Sunday and scattered rain elsewhere.
