The new recruit - now a long-serving police senior sergeant - was at the time a 20-year-old constable with just a couple of years' experience under his belt.
His job at Abbotsford was to provide security for evacuated homes and residents still in the area, as cracks grew around them.
On August 8 he was alone on site, tucked inside a caravan and waiting for another constable to return with fish and chips, when, at 9.07pm, all hell broke loose.
"A guy came running into the caravan yelling, 'She's away, she's away'," he recalled.
He rushed outside flashing his torch into the void, which revealed glimpses of a hillside on the move.
"It appeared that there were houses on trucks, but without trucks underneath them, that were moving at the end of my torchlight," he said.
"There were sparks at times and there was the noise of graunching houses, timber, metal, that sort of thing, as houses started to fall into the hole, which was getting bigger and bigger."
The main danger was live power lines, which snapped as the streets moved and were sparking on the ground as people tried to evacuate.
Once emergency services reinforcements had arrived, a door-to-door search ensued.
One woman in her 80s had to be "manhandled" out, after refusing to leave, while other residents stranded on the north side of the growing chasm were eventually rescued by firefighters.
Once everyone was accounted for, attention turned to finding accommodation for residents who had nowhere to go.
Forty years on, Snr Sgt Benn said some things could have been handled differently - including an earlier, wider evacuation - but he was proud of the emergency response.
"Well, nobody died, so that's a good start. Hopefully, we did some good things to influence that."
