State Transit buses entering Sydney's CBD from across the Harbour Bridge are being diverted to North Sydney station, while traffic entering the city across Anzac Bridge is also affected.

Around Wynyard Station, Clarence Street and Erskine Street were closed for parts of peak hour after buses struck mechanical failures.

The streets have since been re-opened, but the bus lane on Druitt Street, servicing buses entering the city from across Anzac Bridge, is closed after a separate incident.

"It was really really quiet this morning, but then we've had a really busy second half of the peak, and these problems have really exacerbated them," a spokesman from the NSW Transport Management Centre said.

"The city just clogged up."

The first bus breakdown was on Clarence Street at 8.20am. Another bus failed at 8.45am in the same area. Both have been moved.

The third bus broke down at Druitt Street at 9.25am.

State Transit buses and Hillsbus services from the city's north and north-west suburbs are diverting to North Sydney Station to commuters to transfer to trains. Bus tickets are valid on the trains.

About 23,000 commuters enter the city on 590 buses travelling over the Harbour Bridge on the busiest two hours of the morning. About 5500 commuters enter the city on 160 buses across Anzac Bridge.

But even at the best of times buses struggle for space on tight city roads once they have entered the CBD, let alone when three of them break down.

There are also major delays around Broadway and Ultimo due to a crane fire.

Motorists and being advised to avoid the area, and buses using Broadway near City Road are being diverted, delayed, and missing stops.

The crane fire has closed Broadway between Jones Street and Wattle Street in both directions.

Ticket recognition is in place for bus passengers on CityRail trains in the CBD as well as far as Chatswood on the North Shore Line, Strathfield on the Inner West Line and as far as Sydenham on the Bankstown Line.