It is probably Pink Wax Scale insect, they're the right shape, though the colour varies from greyish pink to pink,and this infestation does affect Lilly Pilly. I've  never seen them quite such a glorious shade of lilac pink before though; you may see lots of ants crawling on the tree, which would confirm it is scale -they like the honeydew the scale produces.  Given the size of the tree, it will be difficult to do much about the infestation - the usual course of action is to pick or rub off what you can with a cloth dipped in alcohol or methylated spirits from the woodier parts, and using an oil spray to try to kill the crawlers, see here under Pink Wax Scale https://www.gardensonline.com.au/gardenshed/gardendoctor/pagenumber_6.aspx

You and your tree may just have to live with the infestation I'm afraid, as the plant is so mature and large. Severely affected branches can be cut out if they start to look sick.