The government has awarded Serco $8 million in performance bonuses since it began running Mt Eden Prison four years ago, figures show.
Department of Corrections took over the running of the prison in July after allegations of fight clubs and assault surfaced, and more recently, it announced its contract would not be renewed.
Green Party justice spokesman David Clendon said each year, if Serco achieved its targets, it could get up to $3.1m in bonuses.
In its first year in 2011, the company got almost $1.3m due to "teething problems", he said.
"In the second year, they were paid $2.7m, the third year $2.9m and this last year it's dropped down to $1.1m given all its deductions about breach of contract that we know about."
A large slice of the $8m paid to Serco should be refunded, given the disaster that had unfolded particularly in the past year, Mr Clendon said.
Corrections spokesperson Julie Robertson said the performance-related payout was paid only if all targets set out in the contract with Serco were met.
The contract also allowed Corrections to claim back any fee paid out if the operator's performance was deemed to have failed for that year.
New Corrections Minister Judith Collins said the bonuses were an operational matter and she could not comment.