The only reason I can offer you about the popularity of
Bevan in Australia and New Zealand is it's Welsh origin. Most Austalians and NZers are of British descent so it makes sense for them to choose Welsh/English/Irish/Scottish names. I had a quick flick through the data base and there aren't that many Welsh names that I could say are popular. Bronwen/Bronwyn,
Tegan,
Tristan,
Vaughn, and
Wynne are the only ones jumping out at me as popular. But even then I would say well known rather than popular. Same goes with
Bevan. I didn't think
Bevan Docherty's name was at all unusual, but I have only known a handful of Bevans.
Also, I wouldn't take those comments seriously. It was an anonymous user anyway. I had a quick look at a few Aussie slang dictionaries and couldn't see anything about
Bevan. I certainly haven't heard the term.
I am from New Zealand BTW, so maybe an Australian could answer this question better.