Chemically treating Christchurch's water to make it extra safe for drinking would cost ratepayers more than $100 million, city council officials believe.

A damning Government report in the wake of last year's Havelock North disease outbreak condemned water regulation across New Zealand and called for the universal treatment of drinking water.

But medical experts believe Christchurch, which has long-resisted suggestions its drinking water should be treated, is a "special case" because its water quality is so good.

GEORGE HEARD/STUFF Canterbury medical officer of health Dr Alistair Humphrey believes Christchurch shouldn't have to treat its drinking water.

Council bosses said introducing chlorination would be a "major undertaking" and would cost millions to install treatment systems in the 50 or so boreholes across the city.

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David Adamson, council city services manager, said: "To treat against things like protozoa we would need something like ultraviolet treatment or fine filtration, and to treat against ongoing E coli would need some residual treatment like chlorination.

SUPPLIED Residents in some parts of North Canterbury have had issues with discoloured and "crunchy" water.

"You're looking at a bill of probably $100 million plus, and my engineers have estimated an operating cost of possibly $5m a year to run it."

Adamson welcomed the report for stimulating "interesting conversation" but said the council had "very good measures" in place over risk, including secure deep boreheads and a stringent water quality monitoring regime, and it should be up to the community to decide whether those measures were sufficient.

"I think Christchurch City Council has got some very good practices, both in the construction and depth of their wells and in their monitoring regimes that produce barriers to minimise that risk."

JOHN KIRK-ANDERSON/STUFF Much of Christchurch's drinking water is drawn from deep aquifers, fed mainly by the Waimakariri River.

Recommendations such as re-examining legislation were "long overdue", he conceded.

But he called for consideration to be given to what safety precautions individual water suppliers had in place before any mandatory treatment was imposed.

"I believe that we are doing a very, very good job where we are."

Most of Christchurch's drinking water is drawn from deep aquifers beneath the city, fed primarily by the Waimakariri River.

The untreated water is of an exceptionally high quality and has long been a source of pride for the city.

Canterbury medical officer of health Dr Alistair Humphrey accepted that "in principle" chlorination would improve safety – but warned it could lead to complacency around protecting sources of drinking water.

123RF.COM The Government's report has called for the universal treatment of drinking water.

But the "multiple barriers" the city council already had in place and its investment to protect the supply network were as good as chlorination and should allow it to be exempt from treatment, he said.

"I think the case that Christchurch City Council have made for not chlorinating is a rare but significant exception from the general rule that chlorinating is what you need to do to keep your drinking water safe.

"I accept Christchurch City Council's rationale for not chlorinating – they have invested heavily in monitoring and maintenance of the system and that, in effect, amounts to an extra barrier in protecting the drinking water which is at least the equivalent of chlorination.

"I do not think the council have ever been complacent about their water supply, even though they don't chlorinate it."

The Government's inquiry uncovered a deeply troubling picture of New Zealand's drinking water, with at least 750,000 people using supplies that were "not demonstrably safe".

Mayors and district health boards have been asked to check water meets current standards after the report found quality in 20 per cent of supplies was inadequate.

As well as urging universal treatment, the investigation recommended a new independent drinking water regulator, tightening of legislation and regulation and having larger water suppliers to improve accountability.

The inquiry was triggered by a bacterial contamination in Havelock North last year that left three people dead and thousands ill. The inquiry was advised on October 24 by the Hawke's Bay DHB that it had identified a fourth death that was likely to have been linked to the August 2016 outbreak.

Minister for Health David Clark is expected to update the Cabinet before Christmas of any necessary urgent action.