The only way an aircraft toilet system could leak is if an aircraft is not properly maintained, an aircraft engineer says.

Authorities say they are baffled as to where spatterings of waste that have appeared on South Auckland properties are coming from, but Drury resident Andrew Burns believes the substance he says is human faeces - and which splashed his wife, children, car and home on September 5 - came from a plane.

It was the second such incident at his home, and the Weekend Herald has learned of eight similar incidents, most of which went unreported.

An aircraft engineer said yesterday that aircraft toilet systems could leak if items that were not meant to be placed in them - such as lighters, knives or forks - got caught in the "dump valve".

The dump valve acts as part of the seal to the tank in which the waste is kept after being vacuum-sucked from the toilet.

Sometimes items became stuck underneath the dump valve and caused leaks, but the engineer - who did not want to be named for fear of losing his job - did not believe this happened often.

He said all the airlines he had worked for had been extremely stringent about the extra caps and bungs required to prevent leaking.

He also believed that anything that leaked from an aircraft would appear blue because of the chemicals used in aircraft toilet systems and tanks.

The Civil Aviation Authority and airlines say aircraft toilet systems cannot leak. But their comments have been challenged by a retired aircraft engineer who said he sometimes saw brown trails along the rear of planes caused by overflows when toilet sensors became blocked or faulty.

The Weekend Herald now knows of eight cases in which a brown substance thought to be human waste has spattered properties in Karaka, Drury and Papakura.

Civil Aviation Authority spokeswoman Emma Peel this week said the chances of such a leak from an aircraft would be very unlikely.

"I really hate to use the word impossible, but I would say it would be 99 per cent unlikely."

Meanwhile, Auckland Museum curator of birds Brian Gill said although ducks could be behind the brown spatterings, birds' waste had distinct white speckling - unlike the cases reported.

"A bird dropping has got a lot of white on it ... half of the dropping will be white and the rest of it will be the brown faecal matter ... so if it was birds, you'd expect them to be seeing some of that white matter on the splattering as well."

Nathan Guy, Associate Minister of Transport, said yesterday that it was "very unlikely" the waste was from an aircraft. "The only way it could leak out would be through a major problem with the toilet tank or pipes, which would be very obvious. At this stage there is no concrete evidence the waste was from an aircraft.

- Additional reporting Vaimoana Tapaleao