A damaged pipe led to one of the worst sewage spills a long-time Watercare manager has ever seen, over Auckland Anniversary weekend.

Raw sewage leaked from a large sewerage pipe in Alma Rd in the early hours of Sunday, flooding 13 properties and forcing the evacuation of five homes. A health warning was put in place at nearby Milford beach.

Watercare wastewater transmission manager Chris Harbour said, in his more than 25 years' experience, this is one of the worst wastewater spills he has seen.

Supplied Watercare says the leak at Alma Rd, Milford was the worse in 25 years.

"I have certainly seen much bigger pipes than this one break but, in terms of impact on residents, unfortunately, this is one of the worst incidents I've dealt with," Harbour said.

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The sewage flooded inside four homes and the gardens or grounds of the other nine.

Supplied The damaged pipe.

Following the event, two houses in Alma Rd required interior commercial cleaning. A third and fourth properties required additional cleaning underneath the homes.

Watercare was helping residents to organise professional cleaning, including a "hydro-vac" to deal with the hard-to-reach areas.

Watercare was also arranging for contractors to return to restore bark, pebbles and help tidy gardens.

Supplied The scene at Alma Rd, Milford.

Harbour said, despite the seriousness of the situation, the residents affected were extremely cooperative.

"Not only did they follow instructions and take our advice, they also showed a lot of kindness and generosity to our staff, by providing cold drinks and snacks as they worked away in extreme heat over many hours."

All residents, apart from the two worst-flooded households, were returned to their properties by Sunday night.

Supplied SafeSwim on Tuesday still advises swimmers to avoid the beach, as shown by a "high risk" warning. [Alma Rd can be found in the bottom left part of the map]

Temporary repairs to the broken pipe have made and now Watercare will now move to permanent repair.

Investigations into the cause of the breakage are ongoing but it is believed that possible third party damage, may be to blame.

"This was a strong concrete pressure pipe. These pipes don't typically corrode because they are 100 per cent full during normal operating conditions, with no access to air.

"Therefore that leads us to believe that the pipe has been damaged at some stage, creating a weakness, leading to the pipe blowing out in such a dramatic way," says Harbour.

The Alma Road wastewater pump station has been operating normally since 6pm Sunday night.

Safeswim on Tuesday still advised swimmers to avoid the beach, displaying a "high risk" warning.

Safeswim technical lead Dr Martin Neale said water quality tests performed at 9am on Monday show that water quality remains below recommended guidelines for swimming at Milford.

"As such, water quality alerts advising that it is currently unsafe to swim at Milford Beach will remain in place," Neale said.

"We will continue collecting daily water samples and will be providing updates via the Safeswim website."