If you're after a real-life ghostbuster, Dr Peter Unmack is your man.

The University of Canberra research fellow has led a team of scientists from the university and the CSIRO "ghost-hunting", tracking down a mysterious species of fish in two small creeks near Gunning.

It is a stunning and complicated discovery that proves extinction may not be quite as final as we thought - and that sex is far from simple.

"These are freaky fish," Dr Unmack said of carp gudgeons, one of the most abundant groups of fish in south-eastern Australia.

"They're bloody complicated, so whether someone can wrap their brain around the whole story; that's going to be pretty challenging.

"But I think they can walk away thinking, 'Holy shit, that's really bizarre'."

In rare instances, technically extinct species can leave behind a "ghost" imprint of themselves, intact but hidden within the genetic blueprint of a living relative.

While the ghost species is long gone, its genome lives on.

Scientists had long believed this to be the case in carp gudgeons, a group that includes species that reproduce sexually, like humans.

But the group also includes so-called sexual parasites – fish that mate with a sexual host but only pass on their own genes to the next generation, in a process Dr Unmack said was still not entirely understood.

Previous genetic studies all showed one of the most common sexual parasites, Lake's carp gudgeon, reproduced the genome of a mysterious, unknown sexual species, known as Species X.

“Species X had never been collected, despite extensive survey work,” Dr Unmack said.

“As such, it was thought that Species X was extinct, remaining cryptically alive as a 'ghost' species embedded in the genome of Lake's carp gudgeon.”

But Dr Unmack and his team have laid the "ghost" to rest after tracking down populations of Species X in two small creeks in the upper Lachlan River, in the Murray Darling Basin near Gunning.

"I wasn't expecting to find it," he said, revealing the mystery of Species X stretched back about 20 years.

"Totally wasn't expecting to find it, especially not in Gunning of all places."

A paper describing the former "ghost" is now in the works.

So, apart from carp gudgeons having what Dr Unmack describes as a "broken" biology, why should we care about these "freaky fish"?

It turns out they might be quite important to a larger fish that commonly finds its way onto our plates, and our enjoyment of one of Australia's favourite pastimes.

"They’re probably quite important in terms of food for larger fish and for birds," Dr Unmack said.

"Cod and carp gudgeons spawn around the same time, but cod larvae are much bigger than carp gudgeon larvae, so [cod] can just eat them. They could be quite a key thing to get cod going.

"The other thing is that because these guys live in flood plains and creeks and out-of-the-way places, carp gudgeons are quite important in terms of controlling mosquito larvae and other potential insect pests that people don’t like being bugged with at their barbecues."