This means investors using their property for short-stay accommodation may now be getting less than they could from a long-term lease.

As of September 4, there were 12,083 listings on the accommodation sharing site, up from 8,663 in January, according to data mining site Inside AirBnB.

The increasing supply means listings are booked for an average of 74 nights a year - a 20 per cent occupancy rate - compared to about 110 nights last year.

Average monthly earnings for all listings including a shared room and a private room in a home has dropped from $1246 per month to $847 per month.

Earnings for an entire apartment were now about $1,201 per month, but higher at $1800 in the city centre.

About 5,100 of the listings in Melbourne were for private rooms in occupied homes, but the majority - 6,682 listings - were for entire homes or apartments.

Of these properties nearly 4,400 were available for most of the year. 

When The Age examined Inside AirBnB's data at the start of this year, there were 1,068 apartments in the City of Melbourne used year-round as short-stay accommodation. That number has now risen to 1,347.

"It is much harder to get a conversion (booking) now that there are so many properties on the market," property manger of Beyond a Room, Linton Wood, said. When he first started managing AirBnB properties on behalf of owners, the occupancy rates were up to 95 per cent because of the hype around AirBnB, but competition was much harder now, he added.  Attractive properties with quick-responding hosts can maintain occupancy rates around the industry average of 65 per cent, Mr Wood said.

While the average price for a night's accommodation in the CBD has stayed at $189 per night, the flats booked most frequently have dropped their prices to about $167 per night and earn an average of $2,802 per month, according to Inside AirBnB. This site is run by Murray Cox, a New Yorker motivated by a dislike of the impact the booking site had on his own neighbourhood.

"The data was revealing about how many residential properties were being illegally converted into hotels, and relevant to other neighbourhoods in New York City, or other cities around the world, and so I created the site to easily browse, understand or download the data," he told Fairfax Media earlier this year.

In the Port Phillip area there were currently 832 houses or apartments kept empty for AirBnB listings, and similarly with 450 properties in City of Yarra, 422 properties in City of Stonnington, and 140 in the City of Moreland.

These are flats rented out to short-term visitors instead of locals in need of permanent housing. Many of these AirBnB-specific properties are now sitting empty for an average of 280 nights a year because of the increased supply.

Across Melbourne, the average rents for a long-term lease were now about $1646 per month for a unit, $380 per week, or $1733 for a house. In City of Stonnington the average monthly income for an AirBnB apartment is now $874, or $903 in Port Phillip, and $1295 in the City of Yarra.