Cuckoo spit
It’s Cuckoo spit, named because it appears roughly the same time as the Cuckoos do, Spring.
Also known as Spittlebug froth, it's the protective environment for Froghopper nymphs to live in. Apparently, the froth reduces the risk of dehydration and deters predators as the babies grow. The adult insects are the sap-sucking Froghoppers. Ten points to Dylan Osler who solved the mystery of who had been spitting all over Raakajlim in the northern Mallee.