Planners say Matthew Guy's pledge to release almost 300,000 extra housing lot on Melbourne's fringe if he becomes premier in November could ratchet up the cost of new homes instead of making them cheaper.

And the Opposition Leader's move – designed to lower land prices – would also leave scores of new suburbs in outer Melbourne without adequate infrastructure, senior planners and their professional body said on Wednesday.

More paddocks will become homes under Matthew Guy's plan. Credit:Eddie Jim

The warnings follow Mr Guy’s announcement that a Liberal state government would fast-track the rezoning of 11,000 hectares of farm land – about 290,000 extra housing lots – on Melbourne’s fringe.

He said this would help make more houses more affordable. The price of a housing lot in outer Melbourne had risen from $203,800 in 2014, when Labor won office, to $323,000 this year.