Would-be homebuyers desperate to break into Sydney's booming property market are being warned about "burn and run" developers seeking to cash in on some of the city's fastest growing areas.

The huge demand for property in greenfield areas such as the Hills district is leading buyers to put down deposits before blocks have been registered or, in some cases, before subdivision plans have been lodged with the council, in a bid to secure the land.

Kellyville , New South Wales

But some buyers are effectively losing hundreds of thousands of dollars as the market rises, or are being locked out entirely when their contracts are later rescinded by developers capitalising on the boom by reselling the same sites at higher prices.

"It's a despicable act," said lawyer Bailey Compton of Leverage Solicitors, who has represented owners in Kellyville and who said he was aware of instances elsewhere in the suburb and in Castle Hill and Hurstville.