The NSW Rail, Tram and Bus Union (RTBU) announced the strike on Wednesday evening as a protest against the government's plan to privatise bus services in the city's south and inner west.

It was due to begin at midnight and last for for 24 hours.

However, Transport for NSW applied for an urgent hearing at the Industrial Relations Commission, which was heard at 8pm on Wednesday. A government spokeswoman said the IRC reviewed the evidence and found the strike to be illegal, ordering the drivers to work as normal.

The union did not immediately respond to questions on whether the strike would still go ahead.

Transport Minister Andrew Constance said he welcomed the order from the Industrial Relations Commission and expects the union to comply.

"I hope drivers ignore the union bosses' reckless behaviour," Mr Constance said. "Anyone who takes part in an illegal strike will not be paid.

"I implore all STA drivers to come to work tomorrow, because their customers need to get to school and to work on time."

Mr Constance said contingency measures will still be put in place for Thursday "to ensure our city is not inconvenienced".

When the industrial action was announced, the RTBU said it was a reaction to Mr Constance's "betrayal" of commuters and transport workers in privatising bus routes.

A total of 1200 public bus drivers were due to stop work, affecting four depots which would be privatised under Mr Constance's plan.

The strike would affect bus services that run from the Leichhardt, Burwood, Kingsgrove and Tempe depots, including all school buses in southern Sydney and the inner west.

Transport for NSW said students would be able to travel for free on other forms of public transport when they displayed their Student Opal Card. Parents and students were advised to go to transportnsw.info to see if their route was affected.

A map provided by the union showed the strike would impact a huge chunk of the city, from Sans Souci to Silverwater and from Glebe to Strathfield, encompassing important arterial roads and the Anzac Bridge.

The Transport Management Centre said routes impacted were the 401, 406, 407, 408, 412, 413, 415, 418, 422, 423, 425, 426, 428, 430, 431, 433, 436, 437, 438, 439, 440, 441, 442, 444, 445, 460, 461, 462, 463, 464, 466, 470, 473, 476, 477, 478, 479, 480, 483, 487, 490, 491, 492, 493, 495, 502, 504, 508, 526, L23, L28, L37, L38, L39, M20, M30, M41, X04 and X25.

Routes 438 and 461 would operate a "limited and modified service" along Parramatta Road from 6am to 10am and 3pm to 7pm, in the direction of the peak only.

Those using route 400 from Bondi Junction to Burwood, which is used to travel to Sydney Airport, were advised to catch the train instead.

A Transport Management Centre spokesman said people should catch trains or the light rail, or "make arrangements with employers to work flexibly" to deal with the "significant" disruption.

"Road users and public transport customers across the network are advised to allow plenty of additional travel time during peak periods as traffic is expected to be heavier than usual and patronage on other modes of transport will be increased," the spokesman said.

Chris Preston, the secretary of the RTBU's bus division, said the privatisation announcement which spurred the strike was made despite workers being assured in writing in December that their bus routes would remain in public hands.

He said drivers are "deeply apologetic" about inconveniencing commuters, but they felt compelled to respond to the "outrageous attack" on public transport.

"There is one person to blame for this stoppage and that is Minister Constance," Mr Preston said. "With no warning, no consultation and against explicit undertakings, he has placed the future of 1,200 bus drivers and depot staff in limbo.

"This action is our members' initial response to this outrageous action."

Mr Preston said members of the public should make alternative arrangements to get to work and school, and urged Premier Gladys Berejiklian to intervene in the situation immediately.

"Minister Constance needs to understand that you cannot treat the community and public transport workers with such contempt," he said. "We call on the Premier to intervene as a matter of urgency and put a stop to this attack on our public transport network."