The West Coast was expected to have the "largest and most intense" rain today with a chance of localised flooding, he said.
There would also be heavy downpours in Nelson today with eastern areas also expecting rain.
Rain would move down to the Bay of Plenty today.
"A number of regions have the chance of a heavy downpour today, but conditions are fairly dry with mostly showers and those blustery nor'east winds.
"Many places will be drier rather than wetter today, but the risk for a really heavy sub-tropical downpour is there for some today," he said.
Dangerous swells of up to 5m were pushing the coastline today from Northland to East Cape.
Mr Duncan said people should avoid the dangerous sea in these areas today.