But Detective Chief Inspector Danny Sharkey said it appeared the advertisement was a hoax.

"Obviously we take these types of matters very seriously. Anyone who would consider advertising their one-year-old child would need to be spoken to," he said.

"However subsequent enquiries indicate that it may be a hoax where people are using the advertisement as a scam so people contact them via an email address and they are then defrauded.''

He said similar online hoaxes had appeared overseas.

The baby was listed on Gumtree between a Kelvinator fridge/freezer and a Samsung top loading washing machine, on a page entitled "General stuff for sale and wanted in Sydney". The advertisement has since been taken down.

The same advertisement has been posted in a separate pregnancy chatroom, and one person has replied, offering a "safe, loving home for your child".

The NSW Community Services Department (DoCS) was alerted about the advert through its helpline, a spokesperson confirmed.

"DoCS caseworkers alerted the NSW Police immediately to assist in locating the person who placed the advertisement," DoCS said in a statement today.

"The identity and location of the child is not known and there is very limited information available to the Department at this stage.

"If the classified advertisement is genuine, there is a father out there in the community who really needs support."