Water restrictions will continue in Hamilton today as repairs are carried out on one of the city's major water mains.

Hamilton City Council staff worked late last night to try to fix the eastern bulk water main which suffered a major leak after a "significant" portion of the Waikato riverbank collapsed yesterday morning.

The leak cut the supply to the water main, which services the eastern side of Hamilton, putting the whole city on its highest water alert level 4, meaning water cannot be used outside.

Council incident manager Trent Fowles pleaded with residents to continue conserving water as staff battle to get repairs completed.

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The western bulk water main and water reserves are serving the city in the meantime.

"Our community is doing a great job at reducing unnecessary water usage, and we are asking that they continue to avoid outdoor water usage at this time. Our priority now is ensuring we get our network fully operational, and all going well we would hope to achieve this in the next day or so."

Water supply to some areas was slowly coming back on. Residents may notice discoloured or cloudy water for a short time, but that was normal after pipe flushing and was not a health concern. It normally goes away after running the tap for a few minutes.

The subsidence at the river bank around the pipe area has also had an effect on water systems on the Hamilton Gardens.

"We have staff working with the gardens team to get their supplies operational again, and avoid any disruption for visitors," Fowles said.

"There may be some limited water or toilet facilities at the gardens today and we encourage visitors to check the gardens' Facebook site for updates."

The damaged pipe is next to to the gardens, where the council had heavy machinery and workers. Fowles asked visitors and motorists in the area to take extra care as the repair work was completed.

It was still unclear what had caused the riverbank to subside.

Further information would be released today.