Wellington commuters are being advised to take an alternative route into work this morning after a crash and flooding on State Highway One.

A southbound car and truck collided in a badly flooded area of the highway between Porirua and Tawa about 5.50am.

The New Zealand Transport Authority says the crash site has now been cleared and all lanes are open, but southbound congestion is significant.

It says motorists should expect delays or consider taking a different route.