It found $9 million went through bad investments with another $11 million in grants and administration payments which came in for criticism.
The poor decisions included the trust's late arrival in a collapsing business plan to develop a golf course, a sports academy, residential sections and a hotel near Turangi.
The inquiry, commissioned by the trust, found the trust bought $10.3 million in bad loans on land worth $6.4 million from a bank and a finance company after pressure on iwi members who had given personal guarantees for the debt.
It was described as an "imprudent investment" which breached the trust deed and trustees obligations, the inquiry found.
It also found hopes for collecting on the personal guarantees for the debt were damaged after those who had provided assurances found they would not be chased for the money.
There was also criticism for a loan to bail out a "dilapidated" hotel, of which the iwi is expected to lose $556,000.
The inquiry found only a single-page loan document existed, appeared to have been signed after the loan was made and was unsecured.
Another $2.175 million was lost in a geothermal project which was in financial trouble at the time the money was loaned, bringing questions over whether "prudent" trustees would have made the investment.
There were also payments to contractors and management "without proper process and accounting for amounts which are not adequately justified".
The inquiry found due to such a lack of documentation around the contracts it was unable to find whether trustees had breached the trust deed.
An additional $29 million was paid to individual hapu groups at a time when the trust did not have a proper understanding of its finances.
The inquiry said it raised questions over whether money paid to iwi members - including home heating and kaumatua grants - would have been made if the financial position of the trust had been properly understood.
Tuwharetoa Settlement Trust chairman Dylan Tahau would not be interviewed, but said the review studied past investments and decisions made by its board.
"While some of the previous board's decisions were poor, we don't believe they were made with ill intent.
"However, to test that would mean going to court and our legal advice was that the cost would likely be more than we could recover."
The decision not to go to court saved the iwi the prospect of taking action against the initial trustees, who were also those who had successfully managed the claim to get the settlement.
A spokeswoman for the Office of Treaty Settlements said the process for selecting the post-settlement governance of the claim was led by iwi and agreed during negotiations.
Constitutional lawyer Mai Chen, who has worked with iwi groups on Treaty claims, said the move from settlement negotiations to handling the proceeds of a claim required different skills.
Maori Affairs Minister Te Ururoa Flavell said there were challenges for iwi having to deal with newly-arrived large sums of money.
Poor decisions
• $2.175m lost: Investment in a failing geothermal scheme including "expired and worthless" intellectual property.
• $556,000 lost: Tired hotel propped up with a loan organised by one of its owners who worked at the trust, poorly documented and a decision 'prudent' trustees wouldn't have made.
• About $4m lost: Bad debt of $10.3 million on land worth less than $6.4 million.
