The gates open at 11am for Big Day Out's 12 hours of non-stop music and organisers say the lineup includes the "mother of all celebrations -- Fear Factory's 21st birthday.
The heavy metal band formed in 1989 and commentators say it "consistently forged new territory, proving themselves masters of metal reinvention".
For the $139 ticket cost, music fans would also get a lineup of other acts from overseas, including Muse, The Mars Volta, Lily Allen and Groove Armada, as well as a number of New Zealand acts, performing on two main stages and several small stages.
Children under 15 years would not be allowed into the event for the first time this year.
The concert began in Sydney in 1992 with the first concert in Auckland in 1994. On January 23, Big Day Out is to celebrate its 100th concert, also in Sydney.
For today's concert, organisers have told fans to bring hats and sunscreen. The temperature at Mt Start Stadium was expected to reach a modest 24 degrees.
