The union representing Sydney bus drivers today apologised to commuters for a snap 24-hour strike called for tomorrow, saying it has no alternative after the State Transit Authority walked away from negotiations.

The Rail Tram and Bus Union (RTBU) says award negotiations between bus drivers and the NSW government broke down three weeks ago, prompting drivers to walk off the job from 4am (AEDT) tomorrow until 4am (AEDT) on Saturday.

The NSW government will attempt to block the strike with a challenge in the NSW Industrial Relations Commission today.

"We've been negotiating with State Transit now for about six months to try and get an enterprise agreement," the union's bus division secretary Raul Baonza told Macquarie Radio.

"About three weeks ago State Transit walked away from the negotiating table, so we don't see anyway forward to get someone to talk to us.