A retired aircraft engineer with more than 40 years' experience has lifted the lid on what could be the source of a brown substance which has been spattering properties and people in South Auckland.

Drury resident Andrew Burns has battled for information from Airways NZ and the CAA and is still fighting to be given a list of the planes flying in the vicinity of his property on September 5 when his wife, children, car and home were sprayed with human faeces.

It is the second such incident at his home since 2007.

The Civil Aviation Authority says there is no evidence the faeces came from a plane because it was not tested for chemicals used in aircraft toilets.

The authority and airlines have said the substance cannot come from planes.

They say aircraft toilet systems have pressurised systems that cannot leak, using vacuum systems that suck waste into holding tanks at the rear of the plane.

When the planes have landed, hoses are connected and waste is taken away for treatment.

But the engineer, who asked not to be named, said it was common to find faulty sensors in aircraft toilet systems.

The sensors are supposed to show when a system is becoming full and stop it from working so it cannot be used.

But he said the sensors were prone to failure - often when they became covered by a piece of toilet paper or had a broken cable or connection.

When this occurred, an aircraft toilet sometimes still flushed, sucking any waste into a full tank, resulting in overflows that leaked from the aircraft.

He said the matter was often reported to airlines, but nothing was ever done about it.

Air New Zealand, Pacific Blue and Jetstar do not believe the incident on September 5 could be caused by their planes.

A spokesman for Air New Zealand said the airline's waste system would shut down when the tank became full and the toilet would stop working, but it could not result in leaks.

A Pacific Blue spokesman said it had had only two instances in the history of the airline where sensors had failed.

In both cases, the toilets had stopped working with tanks that were only half full. Jetstar also said leaks could not occur on its fleet.

The cases the Herald has been told of occurred in the afternoon.

The former engineer believes this is consistent with airlines using aircraft for several journeys over a day without emptying the tanks - or from long-haul flights.

The CAA dealt with about 30 reports of a mystery substance spattering properties in 2003, mostly in the Waikato - all of which were blamed on birds.

Information on the United States Federal Aviation Administration website also notes that incidents involving waste from birds had led to complaints from members of the public believing plane toilet systems had leaked.

A CAA spokeswoman said the authority was continuing to investigate the matter.