Videos revealing an inmate "fight club" must be investigated independently at once, the Corrections Association president says.
Beven Hanlon said prison staff were frequently attacked at privately-run Mt Eden jail.
His comment came after footage emerged on YouTube showing gang members engaging in one-on-one fights at Mt Eden as other prisoners cheered them on.
Private firm Serco runs the prison on a contract from the Department of Corrections.
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The fight club videos have sparked calls for Serco and Corrections to be investigated.
This morning, Mr Hanlon said any investigation must be independent.
"It can't be Corrections investigating themselves as they always do," he told TV One's Breakfast show.
There was enough evidence to start an inquiry immediately, Mr Hanlon said.
"Now we've got video footage of it all happening. What more do they need?"