The warning applies to Wellington and Wairarapa, excluding the Tararua District.
MetService meteorologist Lewis Ferris said the rain wasn't looking "overly heavy" until tomorrow afternoon.
"We have the potential for some quite heavy rain and quite persistent rain that doesn't let up until Sunday."
A strong wind watch is also in place for Wellington and the Marlborough Sounds from 1am on Sunday until midday.
South to southeast winds might rise to severe gale in exposed places.
"It's going to put a real dampener on everything," Ferris said.
"This southerly picking up tomorrow and getting stronger on Sunday will drop the temperature right down."
Waka Kotahi NZ Transport Agency reported road conditions would be monitored closely throughout the weekend.
"Drivers are advised that roads may be closed at short notice in order to ensure the safety of motorists.
"Waka Kotahi urges all drivers to take extra care when driving in areas affected by the severe weather conditions, avoid unnecessary travel, and to check the latest conditions before driving."