An investigation has been launched after a mayday call was received this morning, possibly from a plane flying near the Waimakariri Gorge in Canterbury.

Two helicopters were scrambled by Rescue Coordination Centre (RCCNZ) after the mercy message was received on an aviation channel, but no signs of a downed aircraft were found.

An RCCNZ spokesman said a plane flying over the gorge, west of Christchurch, heard the call at 11.20am and notified air traffic controller Airways New Zealand, which told RCCNZ.

Garden City Helicopters pilot Neil Scott said two choppers had flown up the gorge, parts of which were covered in snow.

The aircraft which heard the call had also joined the search, which covered some 1000 hectares.

Mr Scott said the mayday call had indicated the aircraft would attempt to land in the gorge but it it did not give a more specific location.

It was extremely unusual to receive a mayday call these days, and it was being treated seriously.

However, there was always the possibility it was a hoax or a misunderstanding, he said.

The search was called off around 1.30pm after a thorough search of the area came up empty-handed.

RCCNZ said there were no reports of aircraft missing in the area.

The RCCNZ spokesman said it was now being described as a "potential'' mayday call on a "pretty scratchy'' radio transmission.

"But it's better to be safe than sorry,'' he said.

Airwaves New Zealand has inspected radar data of aircraft in the air at the time and haven't identified anybody, the spokesman said.

"Checks have been made to see if there's been any aircraft reported missing and that doesn't appear to be the case,'' he said.

"We're making an effort to speak to the pilot ourselves to clarify what the nature of that signal was, but at this stage the helicopters have been stood down pending any further updates.''

- APNZ