Updated

A skydiving plane that crashed near a New Zealand glacier in 2010, killing nine people, was overloaded because modifications for it to carry passengers were poorly managed, investigators concluded.

The report has prompted one victim's father to claim New Zealand's popular adventure-tourism industry is unsafe — a claim New Zealand's Prime Minister John Key has rejected.

The small plane had been modified from agricultural to skydiving use three months before it crashed near Fox Glacier airfield on Sept. 4, 2010. But owner Skydive New Zealand failed to calculate the proper weight changes before flying the modified aircraft, the Transport Accident Investigation Commission concluded in its report, released Wednesday.

It was 17 kilograms (37 pounds) over the weight limit when it crashed and too much of the weight was at the rear of the plane, investigators said.

A father of one of the tourists on board blamed the crash on lax New Zealand safety enforcement and a legal system that essentially prevents people from suing for negligence.

"To people thinking of going to New Zealand on an adrenaline sport, think twice," Chris Coker of Britain told the BBC. His son, Bradley Coker, was 24.

Adventure tourism is a major economic driver in New Zealand. About 2.6 million people visit the country each year, with one-third participating in an adventure sport such as bungy jumping, skydiving or jet-boating, according to government statistics.

Key's press secretary, Kevin Taylor, said recent industry reviews have improved safety.

"While the prime minister understands this is small comfort to Mr. Coker's family, visitors to New Zealand can be assured that we take safety extremely seriously," Taylor wrote in an email. "New Zealand is renowned for its adventure tourism activities and, while we will always try to mitigate this risk, it can never be eliminated."

University of Auckland law professor Paul Rishworth said New Zealand's accident compensation laws ensure accident victims receive compensation while removing the "lottery" of civil lawsuits. He said the system may result in companies having less incentive to follow safety rules, although the threat of criminal proceedings also acts as a deterrent.

In an interim report on a separate crash, investigators said Thursday that the pilot of a hot-air balloon that crashed into power lines in January had used marijuana. The investigators did not issue findings on whether his drug-taking contributed to the crash, which killed 11 people.

Investigators are recommending New Zealand introduce mandatory drug testing for all drivers and pilots in positions of responsibility, an idea the country's transport agency has so far rejected.