A response to an Official Information Act request filed by the Otago Daily Times revealed an operation was undertaken to remove sewage from Whanganui National Park by helicopter on May 10.
A 350-litre pod was accidentally released by the pilot, which fell into the Whanganui River.
Although the pod was retrieved, some of the contents might have leaked out, an incident report said.
The second incident happened in the Copland Valley, in South Westland, on October 12.
The pilot had been flying out his eighth load of waste, from Welcome Flat Hut to the Copland Track car park.
While flying near Architect Creek, the pilot realised the tank had detached and fallen to the ground.
He turned back around to look for it and when he could not find it he recorded the approximate GPS location.
Upon inspection, it appeared a carabiner had failed. A further two were added to a replacement tank and the operation continued, documents showed.
Doc aviation risk manager Brent Swanson said the Whanganui incident was caused by pilot error.
Instead of pushing a button to talk, the pilot pushed the button to release the sewage tank by accident.
Although councils and iwi were informed, the public was not notified of the incident as it was not determined to have posed a threat to safety.
Since the incident, the helicopter company has separated the two switches on the craft’s joystick, Mr Swanson said.
The carabiner failure in South Westland incident was the result of a fractured weld.
There had been a lot of turbulence at the time, which was not unusual.
"You can imagine the liquid inside the drum goes up a bit and down a bit and puts force on to the equipment, and that’s what we believe happened there," Mr Swanson said.
While some parts of the tank were recovered when it was located a few days later, it largely blew apart in an area of dense bush and some of it was buried by the force of the impact.
The remote area where it landed was not a particularly sensitive ecosystem, and it was a small amount of sewage relative to the size of the national park, Mr Swanson said.
A different carabiner system was now in use and both incidents were reported to the Civil Aviation Authority.
Neither incident had put the public at at significant risk, he said.
Only a small percentage of toilets maintained by Doc were emptied by helicopter, mainly in alpine environments, he said.
