A day after committing to privatise bus services in Sydney's inner west, the NSW government says a much-touted new route to the lower north shore and northern beaches will remain in the hands of the state-run operator.

Preliminary plans for the B-Line double-deck bus service to Sydney's northern beaches, due to open this year, assumed the service would be run by an operator other than the state-owned State Transit.

But a spokeswoman for Transport Minister Andrew Constance said State Transit would run B-Line services because they fell within a "contract area" it still manages.

"There are no plans to have B-Line run separately out of contract area eight," she said.