"I think we're going to struggle. I think we're all going to see a bit of that wet weather this weekend."
Areas such as Gisborne would likely be the last to see the rain on Saturday and the last to clear up on Sunday.
There were severe weather warnings out for Westland and Buller, where 140mm of rain is expected to accumulate, particularly about the ranges. The heavy rain could lead to slips and surface flooding.
On Friday most of the North Island can expect cloud to start developing and rain coming in the evening.
"Nelson to the Bay of Plenty, definitely a weekend to keep a close eye on that severe weather watch," Law said.
He encouraged people to keep a lookout for weather warnings on the Metservice website.
"Of course, it's the rugby Saturday night here in Wellington," Law said, noting those attending the game between the Lions and All Blacks would be battling the weather.
There was a rough prediction of 10-20mm of heavy rain for Saturday, and overall 15-30mm for the weekend. Game day was also expected to be a typical windy Wellington day, with wind speeds of 40-50km/h predicted.
Saturday would have a high of 13C and a low of 10C for the capital, and Sunday would bring a predicted low of 8C.
Auckland could expect 15-25mm of rain, mainly on Saturday afternoon, and a further 5-10mm overnight. Sunday would only bring showers.
Temperatures will mild - 16C expected on Saturday with a low of 11C, and Sunday would bring a low of 10C.
The rainy weather will bring with it slighter warmer temperatures than the country has been experiencing, with only small differences between the highs and lows.
Temperatures
Auckland: highs of 15-16C
Wellington: highs of 12-13C
Christchurch: highs of 11-12C
Dunedin: highs of 10C