In the 1960s Stewart Smith began a one-man crusade, releasing thousands of invasive fish into New Zealand's rivers, lakes and streams. Why? Well it had something to do with communism and a lot to do with childhood nostalgia.

Between the 1960s and late 2000s Stewart Smith went on a one-man crusade, releasing thousands of invasive fish into New Zealand's rivers, lakes and streams.

One kind of fish he introduced is now so widespread it's been declared an "acclimatised species", meaning the authorities have basically acknowledged it is impossible to remove it from the wild.

"The amount of damage he did was incalculable," says science journalist Charlie Mitchell, who wrote a feature on Smith for Stuff.co.nz.

"He could be positioned alongside the people who released stoats and weasels and ferrets in New Zealand," says Bryan Winters, who wrote an authorised biography of Smith entitled That Pommie Bastard.

So who was Stewart Smith?

He was a devout communist with a stubborn streak a mile wide, a conspiracy theorist who spent years locked up in a conscientious objectors camp, and an environmental imperialist dedicated to the cause of "improving" recreational fishing in New Zealand.

Stewart Smith helped infest the Waikato river with koi carp, which are now a major pest species in the river.

Go to this episode on rnz.co.nz for more details