Hundreds of people have rallied outside NSW Premier Gladys Berejiklian's office to oppose plans to privatise Sydney's remaining state-owned bus services.

Union members and commuters were among those who gathered in Martin Place on Monday afternoon to oppose the shake-up, chanting "shame Gladys, shame" and "stand up, fight back".

The state government announced in October it would open all 13 of Sydney's metropolitan bus contracts to competitive tender in the next three years.

This includes the remaining three state-run regions - in the city's northwest, north shore and northern beaches, and eastern suburbs - early next year.

Ms Berejiklian had ruled out additional privatisations just days before the March state election.

Unions NSW Secretary Mark Morey said he believed Sydneysiders had endured enough privatisation.

"The real people who are going to suffer in this are the commuters - the older commuters, the people with a disability," Mr Morey told the rally.

A petition calling for Ms Berejiklian to stop the privatisation was signed by 11,000 people and handed to state Labor leader Jodi McKay.

The petition will be presented to parliament in February and is expected to be debated within weeks of that, Ms McKay said.

"(The premier) went to the election, she said if we were going to do it, we would've told you. She lied," Ms McKay told the rally.

Ms McKay vowed to stop the government's plan, saying no tender had yet been issued and it wasn't too late to change tack.

"This will be a debate we will have on the floor of the parliament and we will not give up, we will stop this privatisation," she told the rally.

Transport Minister Andrew Constance has denied the bus regions were being privatised, insisting the model is actually "franchising".

RTBU NSW bus division secretary David Babineau hit back at Mr Constance's claim, saying it was a "play on words ... nothing else".

"The reality is when you have private companies involved in the provision of public service, it's a form of privatisation," Mr Babineau told reporters.