A coroner's report has concluded two paramedics likely died after one fell asleep at the wheel and their ambulance plunged off a cliff and into a lake near Tofino four years ago.

Jo-Ann Fuller, 59, and Ivan Polivka, 65, died after their ambulance drove over a low concrete barrier and tumbled down a 33-metre high cliff into Kennedy Lake.

The ambulance drove over a low concrete barrier on Highway 4 and plunged 33 metres down into Kennedy Lake, killing both paramedics on board. (CBC)

The report concludes Fuller likely fell asleep while driving the ambulance along a winding section of Highway 4 just before 5:30 a.m. PT on Oct. 19, 2010.

Fuller died from drowning, but Polivka died from blunt force injuries caused by bouncing around the patient compartment in the back of the ambulance.

There were no patients on board and it's possible Polivka was sleeping in the back, a practice forbidden by policy.

The pair started work at 2 a.m. that day, less than five hours after finishing their last shift. Fuller voiced concerns but agreed to come in to work.

New barricade in place

The coroner's report makes no recommendations, but B.C.'s Ministry of Highways has increased the height of the concrete barrier at the site since the accident.

Coroner Matt Brown say he believes the changes were prompted by the investigation into the deaths.

"That is a big win for this investigation, I think. And I recently drove that area myself and saw quite a change in the highway there, as compared to previous years. And certainly if that had been there at the time of the death, perhaps we would have had a different outcome," said Brown.

In a written statement, the BC Ambulance Service says it is "working to change the scheduling information system to ensure staff have adequate time away from work to rest.

Thousands turned out at the public memorial for the pair, including paramedics and other emergency service workers from across Canada.

Google maps: Highway 4 and Kennedy Lake, Vancouver Island.