MetService has issued a Severe Weather Warning, as heavy rain is expected for Southland and parts of the central North Island.

"A very humid and unstable air mass lies over the country today, bringing rain or showers to many areas," says a MetSerivce spokesperson.

"Periods of heavy rain and possible downpours are forecast for Southland, Taranaki and inland parts of Central Hawke's Bay to Ruahine Forest Park, where Heavy Rain Watches and Warnings are in force."

People are advised to keep up to date with the latest forecasts in case further areas are added or upgraded to Warnings.

Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.

Area expected t be affected by the heavy rain are Ruahine Forest Park, Central Hawke's Bay west of Waipukurau and the northern Tararua District. The Severe Weather Warning is valid from 1pm - 10pm Saturday December 17. Periods of heavy rain, with localised downpours are possible, especially in the ranges, and locals should expect 60 to 90mm of rain with peak intensities of 25 to 40 mm per hour.