Ships are being held up at the Lyttelton Port, with ongoing industrial action once again forcing the closure of the container terminal overnight.

Two weekend shifts have been cancelled by the Lyttelton Port Company, as the Rail and Maritime Union campaigns for pay increases.

Union organiser John Kerr says the effect on the port is huge.

He says a Maersk vessel which docks every Friday night would usually be turned around in a day, but it's been extended out to 48 hours.