Most Sydney councils will be forced to amalgamate to form councils of 150,000 ratepayers or more, after the Baird government decided to bulldoze through a controversial plan to create larger councils in metropolitan and some regional areas.

A series of forced mergers, to create 25 councils in Sydney, down from 43, will be announced by the Premier, Mike Baird, on Friday.

He will also announce plans for a new Local Government Act which will include a new power for the minister to appoint a financial controller to a non-performing council deemed at high financial risk.

But the amalgamations will be dealt with through the old act, which means there will need to be public hearings by the Boundaries Commission which will be re-constituted. The process will take at least six months and may require a delay in the local government elections due in September.