Three people have been injured after a shunt locomotive derailed this morning in Auckland.

All Eastern Line trains were cancelled, causing rush-hour delays for commuters, but services are running again between Manukau and Ōtāhuhu to a 20-minute frequency.

KiwiRail said the locomotive came off the tracks near Hannigan Drive, St Johns, about 1.25am, damaging a section of the track including a turnout and set of signalling points.

KiwiRail acting chief executive David Gordon said it was too early to determine the cause and a full investigation would be undertaken.

Three people were injured when the engine derailed early this morning. Photo / Darren Masters

"Regrettably, three of our team were injured – one was treated on site and two were taken to hospital for observation," said Gordon.

Gordon said staff were working to reopen the line as soon as possible.

Auckland Transport continues to advise commuters to take scheduled buses and says buses are accepting rail tickets and AT Hop cards.

People are also encouraged to check the AT Mobile App and Train Line Status for further information.