Venues allow customers to "pay what they want" for a beer until the keg runs dry. Of course, someone could take advantage of the offer and drink approximately 150 beers for free, but that would definitely bring about bad karma. The Karma Keg is based on the 'what goes around comes around' principle and proceeds from the keg go to the venue owner's chosen charity.
Other venues place a minimum amount on a beer ensuring that a fair price is paid for a cold one.
On the whole, it appears that Australians are generous with most people paying a fair amount. Maybe the fear of getting hit by bad Karma is enough to make people pay up.