A crane that collapsed into the main harbour at Milford Sound was lifting almost twice the recommended weight for its size, investigations have found.
The 50-tonne crane, which was working on the enlargement of Freshwater Basin, the sound's main harbour, fell on its side on August 8.
It had been mounted on a barge and the driver was inside when it fell.
Nobody was hurt, but the Ministry of Business, Innovation and Employment's Labour section and Maritime New Zealand started a joint investigation.
Inspectors found the crane was lifting 13.3 tonnes when it toppled over.
The maximum it should have been lifting was 6.9 tonnes.
MBIE Southland service manager John Pannett said it had issued an improvement notice to the crane operators, Smith Crane and Construction, to comply with safe operating limits.
The company's own internal investigation had been thorough and the ministry did not formally investigate or report on the accident, he said.
Smith Crane and Construction managing director Tim Smith said the accident was down to operator error.
The driver was an experienced man with 40 years on the job, and it was a lesson for him to not be complacent, Mr Smith said.
"It was unfortunate but the job went extremely well, really."
The Milford Sound Development Authority's $6 million enlargement project - completed in November - lost a week of work while the crane was salvaged.
Its manager, Andrew Welsh, said it had been concerned about what happened, but there was no damage, no pollution and, most importantly, nobody was hurt.
"I'm sure it was a good lesson learned," he said.
"At the end of the day, it's between the ministry and Smiths."
Environment Southland activated its oil spill response after the toppling, but no oil entered the harbour.
Its on-scene commander during the incident, Dallas Bradley, said the regional council was not concerned about the cause, only with cleaning it up.
The crane was carrying 200 litres of diesel and 24 litres of engine oil, with another 1600 litres of hydraulic oil on the barge.
"It could have been a lot worse," Mr Bradley said.
"There was certainly a lot of concern at the time ... a lot [of oils] could have spilled into the prime entry point for tourists into Milford Sound."
- © Fairfax NZ News
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