An official inquiry into the deaths of eight men who flew for five hours on autopilot though the Australian outback before crashing has concluded it will never be known what happened on the doomed flight.

Western Australia state coroner Alistair Hope, drawing the inquiry to a close on Thursday, said the events of 4 September 2000 would remain a mystery due to a bungled investigation by the Australian Transport Safety Bureau (ATSB).

He said the safety bureau's investigation was poorly co-ordinated and had failed to gather the necessary evidence by not ordering a full toxicology report on the bodies until more than a year after the crash.

The chartered twin-propeller light plane travelled almost 3,000 kilometres (1,860 miles) on autopilot from Perth through north-east Australia before crashing almost six hours later near a mining town in the western part of Queensland state.

Communication lost

Ruling the deaths an accident, Mr Hope said it was clear all the plane's occupants were either dead or incapacitated for most of the flight, but it was impossible to establish why.

According to transcripts from Perth airport, half an hour after take-off an air traffic controller asked pilot Ken Mosedale to verify his altitude:

Mosedale replied: "Sierra Kilo Charlie.. um.. standby" - the last words recorded from the Beechcraft King Air plane.

Over the following nine minutes, air traffic controllers frantically tried to make contact with the cockpit, from which only heavy breathing and electronic noises could be heard before the microphone finally went dead.

The ATSB believe the plane became depressurised, leaving the pilot and passengers unconscious and the plane flying automatically until it ran out of fuel.

The safety bureau rejected the coroner's condemnation.

"While an investigation report into a remote 440km per hour (270 mph) impact crash and subsequent fire which destroyed much of the evidence is always open to criticism, based on its initial reading of the 75 page report, the ATSB does not accept the coroner's criticisms," it said in a statement.

No 'black box'

Mr Hope ordered that all planes should now be fitted with audible, instead of just visible, alarms to warn of cabin pressure loss.

He also recommended an investigation into the merits of fitting all pressurised aircraft with flight data recorders - the so-called "black boxes". Flight data recorders are not required on a plane of the Beechcraft's size.

The plane had been chartered by one of Australia's largest gold producers, Sons of Gwalia, and all seven passengers worked for the company. It was bound for the gold-mining town of Leonora.