Bum Grubs
Worms. Intestinal worms.
Look at that dirty little prick over there scratching his arse, he must have Bum Grubs (ass maggot or arse maggot). He laid a turd and it was crawling with Bum Grubs.
hows your bum for grubs
how are you? or what's been happening? good ol' fashioned aussie slang.
as a "very" informal greeting "hows your bum for grubs?