Tennis fans have been turned away at the gates of the Australian Open after buying tickets from an unsanctioned online ticket resale site.

As weekend crowds pushed Melbourne Park’s first-week attendance past 500,000, News Corp reported that a quarter of tickets sold by Viagogo weren’t valid.

“We’ve tried to educate the public for a number of years not to buy from unauthorised resellers and use our official channels,” a Tennis Australia spokeswoman told AAP on Monday.