Big Brother (Australian TV series) season 5

The fifth season of the Australian reality television series, Big Brother, also known as Big Brother 2005, began on 8 May 2005, with housemates going into the House the day before, and finished on 15 August 2005, lasting 101 days. The season was aired on Network Ten in Australia, and TV-2 in New Zealand with a four-week delay. 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 was announced on the finale that was watched by 2.282 million Australian viewers.

The theme for this season 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 attractive singles that were willing to have sexual relations on camera. The fifth season marked a noticeable change in the portrayal of 'Big Brother'. Compared to earlier seasons, the voice, provided by Nick Colquhoun, adopted a much stricter and more authoritarian tone, often described by viewers as "evil". During this season, Big Brother frequently imposed fines and penalties on housemates for rule breaches, reinforcing a tougher approach to house discipline. This stricter style and vocal delivery continued in the following two seasons, with Big Brother maintaining the same stern tone and enforcement of rules throughout the 2006 and 2007 series.

The season came under controversy after viewers complained about nudity and sexual acts shown on 'Uncut'. Communications Minister Helen Coonan asked the Australian Broadcasting Authority (ABA) to investigate whether Uncut complied with the TV industry's code of practice. It was later found that Network Ten breached the code of practice but no direct punishment was imposed by the Australian Communications and Media Authority (ACMA).