Dozens of Sydneysiders who were trapped for hours in their homes by 500 metres of fallen high-voltage powerlines have been told it is safe to leave.

People living on several streets in Plumpton, in the city’s west, were told to stay indoors after the wires fell on multiple homes and streets on Monday morning.

Fire and Rescue NSW crews discovered the lines after being called to a fire in the roof of a house in the area.

The blaze, which was quickly brought under control, was sparked by the powerlines being brought down, Acting Superintendent Bruce Fitzpatrick told AAP.

Firefighters and police remained on scene while the threat was “neutralised”.

Resident Mario Davaro told the ABC he had been asleep when he heard an explosion and realised the side of his house was on fire.

“The (electricity) line contacted the gas line and it just blew,” Mr Davaro said.

Endeavour Energy crews spent Monday completing repairs to the 132-kilovolt fallen transmission line.

It broke at 6am following a lightning storm and strong winds in the area, the operator says.

“We’ve set up a team to investigate the reason why the line failed,” regional manager Drew Ferguson said in a statement.

“It’s highly unusual for a transmission line to fail and often extreme weather events can be a contributing factor.”

They have told residents “they are safe to leave their homes” but must follow all instructions given to them by energy and emergency service workers.

“People should stay well clear from any fallen power line in the interests of their safety, even if the line is dead,” Mr Ferguson said.

Low hanging wires also held up traffic on the M7 near Woodstock Road, with speed limits reduced throughout Monday.

Power was not interrupted to customers in the area and repairs are expected to be completed by Monday night, Endeavour said.