UPDATED 7.57am: Four trucks are jackknifed on both the Canterbury and West Coast sides of the Lewis Pass.

Crews are on site with a tow truck and more are on the way after the trucks got stuck around 4am.

Transport Agency journey manager, Tresca Forrester, said the trucks got stuck after snow became unexpectedly heavy.

"Two cms to five cms were forecast for Lewis Pass, which is normally over a period of time very manageable. But it came down very quickly."

The road's now closed with snow forecast to fall all day.

Snow's also falling on State Highway 73, Arthur's Pass - which remains open but chains are essential.