A maintenance technician at an Ausgrid facility in a Canterbury substation is suspected of causing the power blackout that struck large parts of Sydney on Thursday afternoon, closing roads and rail lines and triggering another peak hour nightmare for tens of thousands of commuters.

The fault at the Canterbury subtransmission substation occurred at 4.35pm, at the same time an Ausgrid employee was conducting what a spokesman described as ''urgent'' but unrelated maintenance work.

The fault caused a power outage that shut electricity to about 60,000 homes and businesses in the inner west and inner south, suspended services on the Bankstown train line and closed the M5 tunnels at peak hour.

It was not until about 6.20pm that both eastbound and westbound M5 tunnels had reopened and by then traffic had banked up deep into the city.