But their absence is becoming felt more keenly, and a few sightings celebrated, experts are warning: beware of imposters.

This specimen above was photographed last night in Austinmer, where it became just the second Christmas beetle spotted there this year. It joined three other sightings from residents of the Thirroul area in recent weeks - a far cry from the swarming numbers many of us remember as children.

But without expert identification it's possible this shiny scarab was another closely related species.

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The online science resource iNaturalist has been running a citizen science project to track Christmas beetles in an attempt to get a handle on the loss.

People can register and use a smartphone app to record their sightings - from multiple angles if possible to allow for confirmation.

Scientists are alarmed at the Christmas beetle's decline, pointing out if this has become so noticeable, how many species have declined that we haven't noticed?

Chris Reid, principal research scientist at the Australian Museum, has said there were reports back in the 1920s of Christmas beetles "drowning in Sydney harbour" as they weighed down low-hanging branches of waterside trees, such was the weight of their numbers.

A far cry from today - and the reason is likely habitat decline.

"The dual life history provides a clue, Dr Reid wrote on the museum's website.

"The adults need eucalypt leaves, and the larvae need the roots of grasses, presumably native grasses. An important habitat for them, the Cumberland Plain woodland, was once widespread in Western Sydney, but less than 10 per cent remains.

"Sydney is now bulging at the seams ... and Western Sydney has absorbed much of the growth. The beetles' former habitat is now a brick, concrete and tarmac jungle. Christmas just isn't what it used to be, is it?"