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The company issued a statement following last week's accident, stressing the investigation into the incident "will be a source of lessons learned for the future for Fulton Hogan".
Fulton Hogan Dunedin regional manager Grant Sime said the safety of staff was the company's "number one priority".
"The fact there was no injury is something to be deeply thankful for, but it is critical that we use this incident to learn from and prevent any repetition."
The accident occurred on Thursday afternoon, when the roller - with a Fulton Hogan staff member at the wheel - toppled off Portobello Rd, on Otago Peninsula, and came to rest partially submerged in Otago Harbour.
The roller was being used as part of work on the Peninsula Connection safety improvement project - a Dunedin City Council project - when the accident happened.
The driver escaped without injury but the incident prompted a stop-work meeting at the site and the launch of a wider investigation, following two previous accidents involving Fulton Hogan at the site.
Council staff had suggested last week the latest incident occurred when the driver of the roller tried to make a 180-degree turn, only to misjudge the manoeuvre.
However, council infrastructure services general manager Simon Drew, in a follow-up statement, said that now appeared to be incorrect.
Fulton Hogan had clarified the roller was not turning around at the time of the accident, but rather "travelling to the end of the job to park up for the evening".
"The roller was turning out of the centre lane, to travel along the edge lane to park behind other machines parked up for the night [when the accident occurred]," Mr Drew said.
Mr Sime said the investigation now under way would look for "any common ground" between Thursday's accident and the previous two incidents.
The aim was to identify any lessons for the future, he said.
The investigation was expected to take several weeks.