These two are much easier to tell apart than the crested terns …
except they are both the same species, Egretta sancta (sacred egrets so named in 1789 because they were supposedly venerated by some Polynesians). They are found along rocky coasts in the Pacific ranging as far north as Japan and as far south as New Zealand. They are fairly common around the Australian coast except Victoria where they are infrequent and Tasmania where they are absent. There are plenty around Broome.
The popular name is Eastern Reef Egret but they are also called Pacific Reef Heron or any other permutation of the words. They are not the only egret or heron to occur in white and grey forms. In my experience the grey ones outnumber the white ones. The cause of the difference is unknown but stable polymorphisms like this can occur where two forms of a gene (allele) exist for a particular spot on a chromosome and having one of each (heterozygosity) confers an advantage compared to two of the same (homozygosity). The advantage may be due to body colour or it may be due to some other unsuspected effect of the gene combination.
The short greeny-yellow legs distinguish the white ones from other egrets. Distinguishing the grey ones from White-faced Herons should pose no problem if relatively sober.