Sydney could be plunged into darkness for days with thousands of commuters stranded in underground train tunnels and sewage plants in danger of overflowing if a South Australian-style blackout struck the city.

What would happen? Up to 10,000 rail commuters could be stranded underground

Up to 10,000 rail commuters could be stranded underground Sewerage may overflow as treatment plants fail

Sewerage may overflow as treatment plants fail Hospitals could have to cancel food service and some surgeries

The alarming picture was painted by the Energy Security Taskforce (EST) of what might happen if the power failed in New South Wales like it did in SA last September.

While the report, released yesterday, noted a similar blackout is "highly unlikely" in NSW, it also argued there were "significant gaps in knowledge preparation and planning for black system events".

And, in the event of a state-wide blackout, Sydney would be one of the last places where the lights came back on, but more on that later.

According to the EST, power to the CBD could be down for several days.

Up to 10,000 stranded train passengers may need to be evacuated from the city's labyrinth of underground tunnels.

The state's hospitals would come under huge strain if the blackout lasted more than six hours, with all elective surgery cancelled, and hospital food and laundry services for patients scrapped.

Traffic is bad in Sydney at the best of times, but the streets would be worse in a power meltdown. ( ABC News: Luke Rosen )

The state's hospitals are equipped with backup generators to run ventilators, monitors and primary lighting, but diagnostic equipment could not be used and sterilisation procedures would take up to three days.

There would also be no air-conditioning in hospitals, meaning many patients would have to be evacuated.

Fire crews, ambulances and police would also be battling in gridlocked traffic, with all road tunnels shut down and cars crammed onto surface roads.

Light rail services would also stop and ferries — which could have as little as 12 hours worth of fuel on board — may need to be used to move people from CBD areas.

Fuel shortage and sewage spill

The EST said there was concern about the ability to secure fuel supplies in an extended blackout event.

While many essential services have their own generators on site, it is not known how backup generators and fuel supplies would be made available and prioritised during an extended outage.

How to pay for fuel could also be a problem, given EFTPOS machines would most likely not be working.

Remaining forms of public transport, such as buses and ferries would also be competing for fuel.

Water supplies to the city could also be affected. Not all sewage treatment plants have backup power generators and there could be the potential for sewage to overflow.

In previous blackouts, landline telephones have literally been a lifeline.

But since the introduction of the NBN, many landline telephones now require power and mobile phones are of course dependent on their battery life.

Customers with fibre to the node, fibre to the building and fixed wireless would not have operable phones.

South Australia's blackout was triggered by severe storms. ( ABC News: Tom Fedorowytsch )

Why Sydney would be last for lights

If there was a statewide blackout in NSW, Sydney would be one of the last places the lights came back on.

This is mainly because it lies physically at the edge of the network in relation to the main generators.

It is also complicated by the complex network of underground power cables which take longer to bring back on line than overhead wires.

Energy Minister Don Harwin said he doubted NSW would have to face a blackout on the magnitude of the South Australian one, but insisted it was well prepared.

"NSW has a range of generating capacity over a different number of technologies so we are in much better shape for other sorts of energy emergencies," he said.

The good news is the last time a blackout of this magnitude happened was during the winter of 1964.

Chief Scientist Mary O'Kane said that happened in the middle of the night and most residents were unaware it had even occurred.