There are heavy rain watches for Northland from midday Monday; and for Gisborne and Hawke's Bay from Tuesday morning.
The watches could be upgraded to warnings, MetService said, and more areas could be affected.
On Monday, heavy rain was expected to be widespread across the country, MetService meteorologist Ngaire Wotherspoon said.
Thunderstorms were also possible for Northland on Monday evening, she said.
Gisborne and Hawke's Bay's heavy rain watch lasts until 6pm Wednesday.
The government is changing rules to make it easier for lines companies to remove trees near powerlines, to prevent outages during high winds and storms.
The move would ensure the power stayed on, particularly during severe weather events, Energy Minister Simeon Brown said. The changes will come into effect in September.
