Northern Territory Police have received a report that a local resident in the remote community of Gunbalanya was allegedly seen filling in several copies of the same-sex marriage survey and marking them "no" rather than delivering them to their intended recipients.

The same complaint was made to Federal MP Warren Snowdon, who said it was evidence the survey could be "rorted and manipulated" Australia-wide.

West Arnhem Regional Council runs the remote post office on behalf of Australia Post, and chief executive Brian Highlands said he had been "shocked to hear" of the incident and was investigating.

Mr Highlands said he understood that the post office manager "gave some mail out to a lady and that contained some of those letters for the same-sex marriage survey, and this lady started opening them in the post office and filling them in, and somebody saw her and rang the police".

"I think it has been signalled that these surveys going out to Indigenous communities was problematic because you can't ensure mail gets to people," Mr Highlands said.

"It comes to the post office and people come and collect it; not everyone's going to collect it, maybe people depend on family members coming down and collecting mail on a regular basis, but it's a bit impossible to get that at home."

The alleged incident comes after a pile of surveys were reportedly found dumped in a Melbourne suburb; it was revealed that the survey results were visible through the reply-paid envelopes, sparking fears they could be tampered with; and the Federal Police were asked to investigate the sale of survey forms.

Difficulty in getting survey to remote residents

Mr Highlands said the alleged incident highlighted the difficulty in ensuring postal votes and surveys reached their intended recipients in remote Australia.

"The service that Australia Post allows [in] communities is not to the level you're getting in a mainstream town," Mr Highlands said.

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

"Mail gets sorted into pigeonholes in alphabetical order, so if somebody comes in, they collect the mail for their family.

"You can go and tamper with it if it's your family's mail, I suppose, but is it legal? No."

Under the Commonwealth Criminal Code, opening someone else's mail or tampering with it is an offence punishable with up to five years' imprisonment.

In a statement to the ABC, an NT Police spokesman said: "All complaints about the Marriage Law Postal Survey — including any tampering with surveys — should be made to the Australian Electoral Commission."

The spokesman also said that due to the complexities around people authorising others to collect and open their mail, in order to open an investigation into mail tampering a victim would need to come forward to make a complaint, and that had not yet happened in this case.

Warren Snowdon says the alleged incident proves the survey is open to manipulation. ( ABC News: Margaret Burin )

'A ridiculously stupid exercise': Snowdon

Member for Lingiari Warren Snowdon said he was extremely concerned about the allegations, and said they proved that the survey was open to manipulation across the country.

"This is, of course, illegal, and what it does is throw open the potential for this survey to be rorted," he said.

"It's an allegation which, if found to be true, should be most concerning to the Government, because clearly their pointless $122 million waste of public money can't guarantee the validity of the survey result."

He said there was no oversight to ensure that forms were being received by the people they were addressed to, and no way to prove that those filling them out were the intended recipients.

Mr Snowdon said it was "a prime example of what can happen in such a ridiculously stupid exercise".

He said a complaint about the alleged incident had also been made to the Australian Bureau of Statistics, which is running the survey on behalf of the Australian Electoral Commission, but said that organisation wasn't qualified to run the survey.

"The people qualified to do elections are the electoral commission; this is a polling exercise," he said.

"It's not a very scientific polling exercise, because clearly it can be distorted and rorted... and the ABS simply don't have the skill or the knowledge to be able to do it."

The ABS said it had been conducting large social surveys since the 1970s.

"The ABS is familiar with undertaking voluntary statistical surveys that seek the views and opinions of the Australian population about their self-perceived wellbeing, social experiences and society in general, including surveys of a sensitive nature," it said in a statement.

"The ABS maintains processes of high integrity, commensurate with those expected of a world-class national statistical office."

