Union in court over Sydney bus strike

The union which called a last-minute 24-hour bus strike that left thousands of Sydney commuters stranded is being taken to court by the NSW government.

Transport Minister Andrew Constance on Thursday threatened legal action if 1200 drivers ignored an Industrial Relations Commission order to return to work after it had previously deemed the strike illegal.

The Rail, Train and Bus Union says it passed on the commission's order to drivers but wasn't able to force them to return to work before the strike ended at midnight.

The government's transport secretary on Friday morning commenced legal proceedings in the Supreme Court with a view to obtaining a penalty against the union for the alleged contraventions.

AAP understands the union could be fined up to $20,000 if the court finds it did breach the two IRC orders.

The drivers were striking to protest the government's plan to privatise routes in the city's inner west and south where there have been issues with late services.