Rocks dumped on the Greymouth aerodome runway nearly spelled disaster for those on board an air ambulance making an emergency pickup last month.

The Flying Doctors plane was making an emergency pick-up from Grey Base Hospital and only narrowly avoided crashing after its pilot spotted the rocks on July 26, the Greymouth Star reported.

Whoever dumped the rocks the night before would have had to climb a six-foot fence to do so, Greymouth Aero Club spokesman Merv Ashby said.

It was understood the plane's co-pilot had to get out of the plane to clear the runway before they could take off with the patient on board.

Grey District Mayor Tony Kokshoorn said the dumping was a deliberate act and had put lives and property at serious risk.

"It is even worse that the air ambulance was involved. The aerodrome is an essential part of health services in Greymouth and the people who did this need to take a long, hard look at themselves."

A police spokesman told the Greymouth Star the incident was being investigated.