A seven-tonne roller was left on its side with its motor running after vandals turned it on and put it in reverse at a roadworks site in Abbotsford on Wednesday night.
Fulton Hogan regional manager Richard Fulton said staff were doing maintenance pavement repairs in Neil St and had left the roller on site overnight.
During the night, vandals had broken into the steel cover over the ignition and started the machine, which mounted the kerb and went over a bank, tipping over, he said.
"It was a very dangerous thing to do and it was very lucky they didn't harm themselves."
The rollover bars protected the frame from major damage but it was not yet known how much damage was done to the engine.
The roller was now in the company's workshop.
The incident had delayed its work programme in the area, which was "never good".
It was not an unusual incident for the company, which often had to leave large machinery on site.
However, staff tried to leave it in well-lit areas near homes, Mr Fulton said.
Police were investigating.
