Canberra woman Susan Parsons and friend Gillian Richmond explained the monotreme to their American friend over lunch the same day.

"He said 'What was this thing I've seen?'," Ms Parsons said.

"When he found out it was something quite unusual he was thrilled to bits then wanted to know more about it."

According to the New South Wales Environment Department, echidnas are Australia's most widespread native mammal and found in habitats ranging from snowy mountains to deserts.

The mammals are surely one of the world's strangest. The males have a four-headed penis and mate after forming a train of up to a dozen other echidnas.

The females lay eggs and feed their young milk via milk patches rather than nipples. Neither male nor female echidnas have teeth.

Ms Parsons said beach-goers were respectful of the Shelly Beach echidna, who strolled along the beach before retreating into the bush.

"Everyone was photographing him and nobody touched him, nobody went near him, nobody disturbed him," she said.

"Everyone was amazed because no one had ever seen an echidna at a beach - and besides that, you hardly see echidnas."