A remotely operated navy submersible vehicle will help search for a pilot and his helicopter, which is believed to have crashed into a North Canterbury lake.

The helicopter, with Rangiora pilot Michael Graeme Mehrtens aboard, is thought to be at the bottom of Lake Sumner, about 140 kilometres northwest of Christchurch after crashing on Monday.

Police believe they know where the aircraft is, after debris and an oil slick was found, but the lake is 125 metres deep in the area - out of the reach of divers.

Inspector Craig McKay says the submersible vehicle is expected to arrive at Lake Sumner on Thursday and will begin searching on Friday morning.

But he says it's impossible to tell how long it will take to locate the helicopter, or even if it will be located.

"There are so many variables in this operation. We are working in very deep water, and while we are reasonably confident we know the approximate location of the aircraft, there are no guarantees."

It is hoped, however, that the submersible vehicle would be able to move the helicopter into shallower water so it could be recovered.

A Civil Aviation Authority investigator has also arrived at the scene.

Mr Mehrtens, 35, was the only person aboard the helicopter, which was operated by Way To Go Heliservices.

Rob Kittow, spokesman for Way To Go Heliservices, says staff and friends are shocked and the company will be carrying out its own investigation into the crash.