The Independent Police Conduct Authority says an investigation into the termination of Operation Eight, the Urewera Inquiry, has been a huge burden and forced it to use its cash reserves.
IPCA chairperson Sir David Carruthers told a parliamentary select committee on Wednesday the investigation and subsequent report has been a huge inquiry.
Sir David said it has taken years to complete and was extremely expensive.
"That was the largest police operation to have taken place for many years, it's been a real burden on us.
"The investigation and the report's consumed a large part of our resources and this year, we will be going into our reserves because of it at the end of the financial year."
Sir David told the committee its report will be further delayed if the Supreme Court decides to hear cases brought by each of the four people before the court system.