auDA chief executive officer, Chris Disspain, said the new figure "demonstrated Australian business' enthusiasm for the Internet.
"We have strict rules in place, which protect Australian consumers and businesses and ensure the overall integrity of the Internet in Australia," he said.
Disspain said .com.au or .net.au domain name could only be registered by an Australian business, unlike the global Top Level Domain (gTLD) regime, which includes .com and .net.
"Anyone can register a .com or .net domain name. It doesn't matter if you're a legitimate business or not. This is not the case with .com.au or .net.au, which means that Australian consumers can feel confident 'that what they see is what they get'," Disspain said.
Disspain added that .au adds integrity to Australian consumers and businesses because it "clearly identifies them as being uniquely Australian".
"The use of a com.au or net.au domain name, tells the world that you are an Australian business. In many cases an Australian business will be trading on the fact that they're Australian so why be circumspect? Show your colours!" Disspain said.
He added that reaching 500,000 com.au and net.au means that AusRegistry, which manages the database of all registered .au domain names under a licence agreement with auDA, has reduced the price it charges to accredited registrars.
auDA had reduced its registration fee from AU$10 to AU$7.70, effective August last year, after the total number of registered .au names increased by 110,000 to 433,000 between June 2003 and June 2004.