Wairoa District Council is telling residents to be ready to leave at a moment's notice.

A heavy rain watch for Wairarapa and the Tararua Ranges from 9am Tuesday until 6am Thursday is likely to be upgraded to warning levels, MetService says.

Severe gales and heavy falls are set to lash central and southern parts of the North Island on Thursday.

Wairoa District Council is telling residents to be ready to leave at a moment's notice.

Wairoa residents received an emergency alert on Thursday night as floodwaters began to rise in low-lying areas.

'Evacuate now. Evacuate to higher ground or inland,' emergency alert says.

Mayor Craig Little said the council was right to trigger a local state of emergency.

Dozens of residents in the town of Wairoa have been evacuated overnight after the river burst its banks.