The lucky fan will win a new Welsh rugby shirt, a unique pitch side experience plus a pair of tickets for the following three internationals and a selection of Carling branded merchandise.
Carling, brewed by Molson Coors (UK), became the official lager supplier of the Millennium Stadium in 1999 and visitors to the iconic venue have enjoyed the UK's best selling lager sensibly, soaking up the family friendly atmosphere at the home of Welsh rugby, since the beginning.
WRU Chief Executive Roger Lewis said: "Carling has been a major sponsor at the Millennium Stadium for over ten years and it's great that this significant landmark will be reached while Welsh rugby fans are enjoying the best rugby in the world at the 2010 Autumn Series.
"The Millennium Stadium has brought over 300 major events to Cardiff since it was built to host the Rugby World Cup 1999 and this landmark number of pints has been reached across a host of great occasions such as Carling Cup Finals, motorsports and concerts at the home of Welsh sport.
"Welsh rugby fans have played a huge part already with in excess of 4.5 million fans getting behind the Wales team at the Millennium Stadium over 73 games and we are extremely lucky in that rugby supporters are generally passionate and courteous as a group, causing no disruptions to the family-friendly atmosphere.
"It was an enjoyable day when we celebrated the 3 million pint milestone during the RBS 6 Nations clash against Scotland at the Millennium Stadium in 2006 and we plan for another great occasion.
"Having a pint of beer is part of the international match experience for many ticketholders and we are very proud of our supporters and their ability to enjoy the social side of match day while continuing to behave responsibly and sensibly.
"This year's the Autumn Series will showcase some of the best rugby in the world ahead of the Rugby World Cup and I'm sure the fans will not be disappointed."
Martyn Cozens, Molson Coors (UK) added: "We're delighted to be celebrating such an enduring relationship with Welsh Rugby and the Millennium Stadium. At Carling we're committed to getting real fans closer to the game and their teams so it feels right that we celebrate this exciting milestone with a money can't buy prize for real fans of Welsh Rugby and, of course, Carling."
The Welsh Rugby Union and Carling are promising fans a memorable occasion at Wales v Australia with a range of marketing and promotional initiatives planned for the event. Tickets are available for Wales v Australia and all Invesco Perpetual Autumn Series fixtures at www.wru.co.uk/tickets and via telephone 08442 777 888 and to personal callers at Cardiff Arms Store, 8 Westgate Street, Cardiff.