The victim, named locally as Steven Buchanan, was severely injured after the vehicle apparently overturned on to its side near a salmon farm at the Locheport sea loch on North Uist.
Early reports suggest a crane was being used to hoist fish farm equipment, possibly heavy cage nets, on to the back of the HGV.
It is thought the truck had a loader crane fitted to the back when the tragedy happened on Saturday.
The scene, down a single track road at Locheport - about 11 miles outside Lochmaddy - is close to fish farm cages owned by The Scottish Salmon Company, which are anchored in the middle of the sea loch on the rugged east side of North Uist. Mr Buchanan from neighbouring Benbecula, may have been killed after the lift somehow went badly wrong, indicated local sources. He woked for his father's haulage and transport firm.
Three fire crews, two from Benbecula and the Lochboisdale crew who happened to be training nearby, attended the scene but nothing could be done.
Police investigators were examining the location and when darkness fell used temporary lights rigged by fire crew to illuminate the scene late on Saturday night.
Because the man was killed during the course of his employment the Health and Safety Executive (HSE) has been informed.
A HSE inspector is expected to fly to the island to examine the accident scene once the police have finished their investigation.
A Police Scotland spokesman said: "About 1pm on Saturday 5th October 2013, an incident occurred on the B894 in North Uist. The incident involved an HGV, with no other vehicles involved.
"Fire brigade, ambulance and police attended. As a result of the incident the male occupant of the HGV was pronounced dead at the scene. Police enquiries are at an early stage and involve liaison with the HSA."
A spokesman for the Scottish Salmon Company said: "I can confirm that, tragically, there was a fatal accident at Locheport."