A water outage in Porirua has been isolated to two streets, after a burst main caused the asphalt on a road to crack.

A 300mm main burst on Mepham Place around 2:20pm on Saturday.

That led to outages for the city centre, Rānui, and Elsdon.

On Saturday night, Tiaki Wai said the outages had been contained to numbers 1-16 on Champion Street and all of Mepham Place.

Crews had begun excavating the burst water main.

"At this stage, there is no estimated timeframe for full restoration, but it is anticipated that service will be interrupted for at least eight hours," mayor Anita Baker said earlier in the day.

Pātaka Art + Museum, Café Kaizen, Porirua Library, and Te Rauparaha Arena and Aquatic Centre all closed as a result of the burst water main.

Tiaki Wai expected water to return from around 10:30pm on Saturday night.

A water tank was ordered for affected residents. Earlier, the confirmed it would be located on the corner of Mepham Place and Champion Street.