"Sophisticated" illegal rooming house networks will be targeted in a crackdown by Melbourne City Council, with a professional investigator to be hired to gather evidence against "unscrupulous" operators.

There is growing evidence that city apartments are being crowded with foreign workers, students and backpackers, who are charged excessive rent for tiny sleeping quarters including partitioned living rooms.

"We can see the problem is not improving itself and some serious action needs to be taken," Councillor Richard Foster said.

Melbourne CBD housing advertised on Chinese-language Facebook page. Source: Facebook.

The council has identified at least two "organised" groups renting out beds in a number of illegal rooming houses across the city.