When a long-time Mona Vale resident went to throw the trash into the bin of his apartment block on Friday morning, he lifted the lid to find a bag in it filled with mail addressed to nearby residents.

The plastic bag, which the man found in the red council bin of his apartment block in Seabeach Avenue, contained 39 opened same-sex marriage postal survey envelopes, with the voting forms and return envelopes removed.

Also stuffed in the bag were about 45 pieces of personal mail, all torn cleanly in half.

"They must have just raided all these letter boxes," said the 66-year-old man, who wanted to remain anonymous.