The oldest male platypus ever found in the wild in Australia has been discovered in a Melbourne waterway.

The 21-year-old was found during a survey of waterways in Belgrave last year with two young males, indicating reproductive success, Melbourne Water said on Wednesday.

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Melbourne Water’s urban platypus program has run since 1995, partnering with research group cesar in 2007.

They did not say in their statement how the age of the old platypus was determined.

The survey of 16 waterways over Spring 2015, found 21 of the egg-laying mammals – two of them entangled in litter.

Platypus ecologist Josh Griffiths said litter was a big threat to Melbourne’s platypus population.

“Litter continues to be a major problem for platypus, with animals that are entangled enduring cuts, abrasions, a reduced swimming and foraging ability and, finally, a slow death from starvation,” he said.

With Clean Up Australia Day looming this weekend, researchers want to remind people to be extra vigilant about rubbish disposal.