Denmark is the gold standard for chlorine-free water, but it would take 20 to 30 years for Hawke's Bay to build water infrastructure that rivals it, an expert says.

Water NZ's principal water quality adviser, Jim Graham, said it would be possible for the region's councils to make water chlorine-free again, even after the Havelock North water crisis.

But to do it will come at a cost.

Graham presented a workshop at the Hastings District Council on Thursday about how chlorine-free water supply in Hawke's Bay and nationwide can be achieved by using Denmark and the Netherlands as examples.