A possum caused a power outage to 12,000 homes and businesses on Sydney's north shore this morning, an electricity provider says.

Ausgrid said the possum came into contact with two live parts of the electricity network near a large substation at Killarney Heights and acted as a conductor, creating a short-circuit and triggering the network's safety mechanism about 4.30am, a spokeswoman said.

The power lines between the large substation and smaller substation carried 33,000 volts, and power was interrupted in homes and businesses at Frenchs Forest, Belrose and Killarney Heights.

It was restored to 6000 Ausgrid customers at Belrose and Frenchs Forest at 7.45am and to 6000 other customers just before 8am, the spokeswoman said.

"There are a lot of possums in Sydney and they use the power lines as their own highway - as a connection between trees or roads," Ausgrid spokesman Anthony O'Brien said.