The source of a sewage leak that led to six houses being evacuated on Auckland's North Shore has been found, Watercare says.

Photo: RNZ / Nikki Mandow

A warning was also issued for swimmers on Milford Beach.

Watercare acting chief operations officer Mark Bourne said the focus this afternoon was a replacement pipe, and cleaning up affected properties.

Two properties on the corner of Alma Road and Shakespeare Road had sewage flowing right through the house, and a handful of others had contaminated water in the garden.

Mr Bourne said excavations down to the broken underground sewage pipe - a main line between the Alma Road pumping station and Rosedale Wastewater Treatment Plant - had to be done very carefully, because of Vector power cables close by.

Once the pipe was replaced, the company would conduct an investigation to find out why the 450mm diameter concrete pipe broke.

Watercare has 8500 kilometres of sewage pipe in Auckland and sections of pipe have an approximately 80-year life span, Mr Bourne said.

He said people in houses where only the garden was affected should be able to return home later this evening or tomorrow, but those in the two most damaged houses might be out for one or two more days.

They would be put up in local motels until then.

Unaffected residents of the quiet Milford Street were sympathetic, but not surprised. One man said infrastructure was increasingly stretched in the suburb because of infill housing. Another said that the problem of sewage overflows affecting Milford Beach was not new - his grandmother had warned him not to swim there at certain times almost 60 years ago.