Uncertainty about the future of councils in Sydney and NSW that have not yet been merged could continue into the next decade.

The creation of 11 new councils proposed by the Baird government has remained up in the air this year after some councils that would have been merged challenged their amalgamation in court.

Some of those councils have been partly successful, and some have not. But unless the government moves fast to secure the mergers, elections for the new local government areas may be delayed until 2018 or even 2020.

The Office of Local Government has written to councils that have not yet been merged, telling them that unless they were amalgamated by the end of this month, elections could not be held until March 2018.