AUSTRALIAN police gave Ukrainian cops a vital tip-off which led to the arrest of a couple who allegedly filmed themselves repeatedly having sex with their four-year-old daughter.

According to Ukrainian media, the man, 29, and woman, 30, who are understood to be cousins, then allegedly sold the depraved footage online for as little as $70 in cryptocurrency to sick customers in Australia and Asia.

Local police footage shows the couple being detained in the eastern European country and cops seizing videos, pictures and sex toys.

According to TSN channel in Ukraine, Australian investigators spotted an identifying code in a sick video featuring a four-year-old girl in explicit sex scenes with her parents.

The code drew police in Europe to a chain of shops in the Poltava region of Ukraine.

But cops couldn’t find the couple in Poltova.

“The Australians kept pushing their Ukrainian colleagues — (urging them to) search more — because new videos with some unknown little child were repeatedly being uploaded from the same source,” reported the Moskovsky Komsomolets newspaper.

A police spokesman told Ukrainian media the couple are “pure evil” — adding that they sold the footage for access fees ranging from $70 and $140.

Police eventually traced the couple hundreds of kilometres away from Poltava in Kryvyi Rih, a city in the Dnipropetrovsk region of Ukraine, where they routinely moved every three months.

Dramatic police footage of the arrest shows cops interrogating the male suspect in a raid, while the woman hugged her daughter on a bed.

After a hospital check-up the young girl was taken into care and sent to a rehabilitation centre. The parents are being detained for two months pending further investigations.

Lawyer Ekaterina Malykhina said the girl was distressed.

“She was in a bad way, she was lost, she did not talk to anybody,” she told Ukrainian media.

“Now she is talking to children, she made some friends and trusts them.

“When we heard for the first time about the four-year-old girl being exploited by her parents, I ordered the head of the local police department to stop all other work and investigate this as soon as possible …

“Those beasts have been arrested by police in Kryvyi Rih. There is very little that surprises me, but this is the ugliest possible crime, against a child.

“How could they do it to a dependent child?”

It is not yet known whether any Australians have been arrested for accessing the footage.

A spokesman for the Australian Federal Police force told news.com.au it is not yet clear which state the tip-off came from.

It is understood that more people might be involved in the operation and a search is on for an accomplice.