At least seven apartment blocks in one Canberra suburb have seen same-sex marriage postal surveys strewn about the street by wind and rain, after apparently being left on top of mailboxes, rather than inside them.

Dozens of postal survey envelopes in the inner-Canberra suburb of Braddon were spotted on the ground outside seven different apartment blocks on Saturday morning.

Some of the envelopes had been damaged by rain.

While the letters spotted by the ABC have been returned to their proper mailboxes, it is not known if the surveys inside sustained water damage.

Australia Post says it's confident all surveys were originally delivered securely. ( ABC News: James Fettes )

A bank letter was also found torn open at one of the apartment blocks.

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

It is not clear whether the envelopes were delivered on top of the letterboxes, or inserted halfway into the correct slots, then removed by a third party.

In response to questions about Australia Post delivery methods, a spokeswoman said the local branch was confident all surveys had originally been delivered securely.

She said delivery drivers had received strict instructions to only deliver the surveys to their intended recipients.

"Our posties are delivering all survey items securely and as addressed, as they do with all mail items," she said in a statement.

"Theft or tampering with mail is a criminal offence and carries serious penalties.

"If anyone suspects mail tampering or theft has taken place, we recommend they contact local police immediately."

People with missing surveys can contact the ABS. ( ABC News: James Fettes )

The discovery comes after revelations that responses to completed surveys were visible when a torchlight was shone through the Government-supplied return envelope.

That sparked concerns over the security of the process, however the Government said a thorough review had been conducted before the survey was sent out.

The Australian Bureau of Statistics (ABS) said it would work with any residents whose surveys were damaged.

"Anyone who has damaged or spoilt materials can order replacements straight away via our website or by calling the information line," a spokesman said in a statement.

The ABS said anybody who has not received a postal survey form by September 25 should contact it, in order to arrange a replacement.