More than 5000 households were without power in West Auckland tonight after a substation failed.

At the height of the outage, some 10,000 houses were without power after the Hepburn Rd substation failed about 7.15pm.

By 8.30pm that number had been reduced to around 5500 after power was restored to Henderson Valley, a Vector spokeswoman said.

Crews from Vector and Transpower were yet to find out why the substation failed.