Some of the letters include an offer of a free MP3 player upon registration, according to auDA. However the regulator said registrants had complained they had not received the players, and some net.au domains had not been registered.
The AU$225 fee is also "significantly higher" than the prices charged for a net.au domain by accredited registrars, auDA said.
The letters and faxes were sent by Domains Australia Pty Ltd, a company controlled by Blair Rafferty. He is the brother of Chesley Rafferty, who auDA prosecuted two years ago over similar activities.
Anyone affected by the scam should contact the Australian Competition and Consumer Commission, auDA said.