An ambitious plan to repopulate city streams with whitebait has been launched in Counties Manukau.

And it's hoped it will spread throughout the entire Auckland region with volunteer help.

The Where's the Whitebait? team has been hard at work finding the "tidal wedge" of different streams - the spot where the salt water of the highest tide mixes with the fresh water.

That spot is crucial for native fish because it's the only place where they can spawn, project volunteer Kate Loman-Smith says.

"So if you don't have that one place, you can destroy whole catchments of fish," she says.

The focus is on making the tidal wedge a suitable place for spawning.

And it's a project the Auckland Council is happy to support.

"They're keen to make sure the bank adjoins a slope and make sure it's planted up so the whitebait can lay their eggs in the long grass," project co-ordinator Julia Tuineau says.

"That would preferably be with native plants, but any rank grasses will do."

The outlook for New Zealand's native fish species is dire if human intervention is not taken, Massey University freshwater scientist Dr Mike Joy says. He predicts a number of the country's native fish will be extinct by 2050 if we continue on the path we're on.

"You hear all the time about the biodiversity in New Zealand and that it's declining," Ms Tuineau says.

"Our native birds, like the kakapo, get a lot of protection.

"Our eels and inanga are declining too - they just don't get as much attention because they're not as charismatic."

Forest and Bird South Auckland committee member Dene Andre says fish life has declined drastically in recent years, particularly in urban areas.

"Giant kokopu are a great example - prior to the Europeans coming here they used to be quite common. Now they're only found in a handful of streams in a few areas," Mr Andre says.

"In some areas you're actually going to end up with localised extinction of native fish."

Where's the Whitebait? volunteers recognise the project is a big one.

It involves looking at physical barriers preventing fish from reaching the areas they need to thrive - such as the Otara Stream's concrete dam.

"Removal of a structure like that is really costly and there will be environmental impacts," Ms Loman-Smith says.

"There are a lot of different arms that need to work together for this to be resolved."

Volunteers are urgently needed for the project to spread. And it's set to pay huge dividends for local communities.

"For groups that are already looking after streams, this is an extra string to their bow - they can protect the native fish by protecting their spawning area," Ms Tuineau says.

"And the spawning area is usually around where people live so if we can get people to look after that area of the stream, they can have whitebait in their backyard."

Contact Ms Tuineau on 021 264 7100 to volunteer for the project or for more information.

- © Fairfax NZ News