Victoria University freshwater scientist Dr Mike Joy says New Zealand is at risk of "killing the golden goose'' if work is not done to restore its waterways.

At a public forum in Queenstown attended by more than 100 people last night, Joy said 74 per cent of the country's freshwater fish were now "threatened species'' and if nothing changed, by 2050 all "will be gone''.

New Zealand boasted the cleanest lakes in the world but also had the dishonour of having some of the most polluted.

He said the country "went wrong'' by intensifying development to the point of harming waterways and it was not possible to have a healthy economy without a healthy environment.

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"We have to have that [and] we have to take that seriously.

"More intensification, more people, more tourists - more impacts come from that.

"We are really at risk of killing the golden goose - that clean green image is so valuable and if we lose it, we will never get it back.''

Queenstown Lakes Mayor Jim Boult gave an assurance the Queenstown Lakes District Council and Otago Regional Council would work more collaboratively to try to resolve issues.

"It's not an ORC [or] QLDC issue; it is an "us" issue.

"It will come at a cost, a physical and financial cost, but we can't ... continue to watch water quality deteriorate.

"It is simply unacceptable.

"We can't change the past, but we sure as hell can change the future and I'm determined that our council can and will ... do something about the future.''

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ORC chairman Stephen Woodhead agreed and said it was time for the "debate around water quality to mature''.

"A lot of work has been done ... but there is still a long way to go and the next stage is going to be hard. It relies on us changing behaviour.

"I'm very happy to make a promise that ... we are working to make it [better] over time.''

Last night's forum was organised by Crux.