Greymouth residents are being warned about a possible door-to-door postcard scam.

Police Sergeant Paul Watson said a member of the public had reported a suspicious man selling postcards.

"The person is going door-to-door trying to sell postcards. We want to urge people to be mindful if they are approached in their homes or businesses by someone selling postcards," he said.

"They need to check if the person has ID from the organisation they say they are selling for and, if they don't, I'd be very wary of making any purchases."

If residents had any concerns about suspicious behaviour they should call police, he said.