A woman was badly injured in a car crash at Porirua this morning as police in the Wellington district warned about tricky road conditions due to heavy rain.

The woman's car left the road and smashed into a tree in Warspite, Avenue, Porirua, just after 5am.

She had to be cut free by firefighters and was taken to Wellington Hospital where she was reported to be in a critical condition.

Police said that the road was partially open but delays may occur as commuter travelled to work.

Meanwhile police in the Wellington area urged drivers to slow down, increase following distances, and drive to the conditions.

Sergeant Mark Oliver said the roads were very greasy in places, particularly the Ngauranga Gorge, as a result of the heavy rain.