Commuters are being encouraged to plan for potential delays this morning after a tree fell on the Kapiti rail line.

The large macrocarpa fell on the tracks near Takapu Road Station in Tawa yesterday afternoon, trapping passengers and knocking down powerlines which sparked a scrub fire.

KiwiRail is replacing the trains between Porirua and Wellington and the Capital Connection with limited bus services today.

NZTA spokesman Mark Owen says with reduced public transport capacity, they're expecting more people to take the car this morning, resulting in more congestion on the highways - particularly State Highway One.

He says travelling outside of peak times will help, so people should either come into work earlier than usual, or text the boss and say they'll be heading in a bit later.