Another sellout crowd of 45 000 is expected to cram into Auckland's Mt Smart Stadium on Friday for the annual Big Day Out and St Johns Ambulance is urging those attending to stay hydrated.
Temperatures of up to 25 degrees are expected, meaning concert goers need to be aware of what the heat can do.
Event Manager Charlotte Guscott says this year St Johns has paramedics keeping an eye on the mosh pit.
She says every year several people collapse from heat exhaustion there.
More than 130 St Johns staff are available to those seeking medical help.
The music festival has become a fixture on the New Zealand music calendar since its inception in 1994.
Headliners Rage Against The Machine are the draw card for many 20-somethings who grew up chanting to their political and often angry lyrics.
The American band plays the main stage just after 9pm tonight, following on from Icelandic sensation Bjork, who aside from her eccentricities, delivers a powerful live show.
New Zealanders Shihad get a prime spot for their new material, they play at 7pm tonight, after critically acclaimed Canadian indie band Arcade Fire.
Supergroove also play today, for the first time since 1995.
The reformed group will close the music extravaganza at Mt Smart Stadium tonight.
They'll play on one of the smaller stages after Rage Against the Machine wraps up the acts on the major stages.
Supergroove member Nick Atkinson says playing the 1995 Big Day Out was chaos.
Among the other New Zealand bands playing the Big Day Out are The Clean, Liam Finn and The Checks.