What the hell are those?

are they cicadas?

IF they are Christmas beetles:

"Christmas beetles are known for causing damage to plants in gardens and yards. They are particularly fond of eucalyptus plants, but may also target other garden plants. The damage usually appears in the form of chewed leaves, with the insects consuming the soft, green parts and leaving the tougher veins behind."

June beetles in the USA do the same thing.

They resemble June beetles, which are natives of the United States, but they might also be a variety of Christmas beetle which is native to New South Wales.

They look like the larval form of something. Kind of resemble aphids, but they don't look quite right to be aphids. Can you tell what type of damage they do, are they eating the leaves?

Uge, I guess some kind of leaf eater - I would try spraying them with some water mixed with a good dose of dawn dish soap. Maybe a little neem oil too.