Cellphone distraction appears to have been a factor in a locomotive tumbling into Picton Harbour last September

A Kiwirail internal report released to Newstalk ZB shows a remote control operator failed to set the brakes on unmanned shunt locomotive, before walking away to use his cellphone.

The 50-tonne locomotive and a wagon containing hazardous materials ended up in the harbour.

The wagon carrying sodium hydroxide and the liquid nitrogen floated in the harbour until it was pulled out hours later.

It took another two days to retrieve the locomotive from the seafloor using specialised airbags.