A new stocktake has revealed drinking water supplies in some of New Zealand's main centres have failed to meet a key safety standard.

The Ministry of Health's latest drinking water report assessed all networked supplies serving more than 100 people between July 2017 and June 2018.

It found 97.7 per cent of 3.8 million people were getting water that met bacteriological standards - up 1.5 per cent on the previous year.

But there was a big variation when it came to size of supplies.

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Standards were met for 95 per cent of people served by large supplies, but

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'THIS IS AN ONGOING PROBLEM'

BY THE NUMBERS