Commuters have been warned to expect major disruption and delays to Sydney's public transport and on roads during Thursday despite a planned bus drivers' strike being ruled illegal in a late-night Industrial Relations Commission hearing.

Hundreds of bus drivers failed to turn up to depots in Sydney's inner west and south early on Thursday morning, which the Transport Management Centre said would have a "severe" impact on the morning commute across the city.

The drivers' absence comes the morning after Transport for NSW applied for an urgent hearing in the Industrial Relations Commission to challenge the planned strike, which was due to start at midnight on Wednesday.

The hearing was held at 8pm on Wednesday. A government spokeswoman said the IRC reviewed the evidence and found the planned strike to be illegal, and ordered the drivers to work as normal on Thursday.