Hamilton residents are on the highest water alert level after one of its most important water mains ruptured when a large portion of the Waikato riverbank collapsed.

Hamilton City Council waters unit manager Maire Porter this afternoon urged all of the city's residents to immediately begin conserving water after a "substantial section" of the Waikato riverbank collapsed around the Eastern Bulk Main, causing a "significant" water leak on the site.

The leak means the whole city is put on the highest water alert level - 4 - which means there can be no water used outside.

However, Porter was quick to reassure residents there was nothing wrong with the water quality.

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"There are no issues with water quality but supply levels could be reduced significantly due to the damage. Alert Level 4 means no outdoor water usage is permitted, but residents are being asked to look at all water usage in their homes and businesses as staff work to repair the Eastern Bulk Main."

The Eastern Bulk Main supplies around 27,000 homes and runs under the Waikato River from the city's water treatment plant opposite Hamilton Gardens.

A substantial section of the riverbank has collapsed around the Bulk Main and there is a significant water leak at the site.

City Waters staff have been on site this morning making the area safe to work in, and identifying the extent of the breach and repair options, she said.

A council spokesperson said in a best-case scenario the leak could be repaired in five to six hours, however until staff can get into the pipe to assess the damage, they couldn't be sure of that.

As for what caused the riverbank to collapse, a council spokesperson said there was no work going on in or around the area at the time, so it was currently a mystery.

Porter said until the pipe can be repaired, water will be supplied to the Eastern side of the city via the Western Bulk Main and from water stored in reservoirs.

"This means a concerted effort will be required from the whole city to maintain supply.

"Until we complete this repair we need everyone to play their part. This is a time when our city can come together to help each other out and we can get through this situation as smoothly as possible.

"We have a team of expert staff who are working to resolve this situation as quickly and efficiently as possible. We will be providing regular updates on the council's Facebook page and through media, however it would help get the message out if people can let their friends and neighbours know."