Was checking if Coles or Woolies was half price this week (it's Coles) and instead big w blew them both away.
Only looked at men's trunks, but found guy fronts, total packages, fit, xtemps etc in various colors and sizes for $3 each.
Quick search shows women's and kids too, and socks and others.
These are clearance so get em while they're hot
Search link in post, will try to add a few direct links later but this'll have to do while I'm on mobile.
Edit: As stock seems to be low, I'll also throw in a mention for the Bonds outlet store, which seems to have trunks for $6 or $7 plus del. Haven't investigated, but good luck to those itching for a new pair. Not as great in comparison, but would have been deal worthy on its own.
https://www.bonds.com.au/outlet/men/underwear.html
And while on the topic, the Kmart 7packs for $20 are a good regular option.
A really great find, bought lots my sizes for pick up at local store. Not many colours of choice & just hope their stock isn't problem now!