"It's not looking very nice. The Wairarapa is expecting some of wettest weather ... it's not going to be a pleasant place to be over the next few days," he said.
"There will be heavy rainfall with strong winds - some gail force and gusty in spots."
Late Thursday, the pressure system bringing the wild weather was expected to "pull away" from the east to the west, easing the pressure, he said.
Mr Law advises travellers to keep an eye on the weather and be advised by the New Zealand Transport Agency website about road conditions.
Yesterday, contractors were clearing away two slips on the Wellington side of the Rimutaka Hill.
Mr Law said the wet weather expected could cause more slips on roads.
"Something for travellers to bear in mind is slips along with taking extreme care in exposed places."
The recent wild weather was not unusual for this time of year, Mr Law said.
"Spring is a time for change ... just with this one coming from the north, it is bringing with it a lot of heat and moisture."
MetService meteorologist Daniel Corbett said staff would be keeping an eye on the situation as there was a "heightened risk of flooding" especially across eastern regions.