Big Brother Australia 2005, also known as Big Brother 5, was the fifth season of the Australian reality television series Big Brother Australia, and was aired on Network Ten in Australia, and TV-2 in New Zealand with a four-week delay. The series started on 8 May 2005, with housemates going into the House the day before, and finished on 15 August 2005, lasting 101 days. The theme of the series was "single, sexy, and competitive". Auditions for housemates were held in March 2005. In a departure from usual procedure, candidates were not required to send in videos of themselves as had been the case for prior auditions. Instead, the producers toured major Australian cities and conducted interviews. They searched for sexy singles that were willing to have sexual relations on camera. Promos for the show suggested that Big Brother would be different this year, and phrases such as "Assume Nothing, Expect Anything", "Let's Play", and "Think Again" were used throughout the series, especially during Opening Night. The winner announced on the finale was watched by 2.282 million Australian viewers.
On Opening Night, it was revealed that $5,000 Australian dollars would be subtracted from the $1,000,000 prize money every time the housemates broke a rule. Such rules included discussing nominations, which must remain secret, not giving adequate reasons for nominating, and forgetting to wear microphones at all times. Michelle was known for being the housemate to get the most such fines - $65,000. Geneva had the least amount of fines $15,000, two in the early days at the house and one near the end of her eviction.
It was revealed that twins would be entering the House, regularly changing places without the other housemates knowing. Greg and David entered the House as a single person named Logan (their shared middle name). Their task was to remain undetected for two weeks while swapping places at Big Brother's command. The secret of the twins was discovered one week into the series by housemate Glenn, and this was revealed during Lies Exposed.