The net is closing on an anonymous writer with a "poison pen" sending letters to residents of a small Otago town.

Senior Constable Helen Fincham-Putter said three anonymous letters about Middlemarch residents, have been handwritten with a "poison pen".

"We have a right little busybody in town."

Two letters were sent to two separate married women and gave "unwelcome advice" on the "state of their marriages".

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"If you think someone needs to know that information, you should be able to tell them face-to-face. Otherwise, it's just gossip and you should just keep your beak out," Snr Const Fincham-Putter said.

The other letter sent to Mosgiel Police Station listed alleged disqualified and unlicensed drivers living in Middlemarch.

The letters were posted "out of town" but Snr Const Fincham-Putter was sure the writer was from the town.

"They know stuff, more than anyone from the outside looking in - that's for sure."

A police forensics team said 90 per cent of the time the letter writer was a woman.

"Guys usually wouldn't sit down and go to that trouble to write a letter - the average guy would just say it."

Snr Const Fincham-Putter was trying to match the letter with handwriting in condolence books from funerals in Middlemarch.

"We may not know today or tomorrow but it will come out one day because someone will say something and it will all click."

If anyone got a similar letter they could talk to a trusted friend.

Or the letter recipient could ignore it because if the writer "can't be bothered putting their name to it, I don't give it much credence," Snr Const Fincham-Putter said.