Darwin's waste dump seems an inglorious full stop to an exciting story involving Russian helicopters, mercenary special forces soldiers and the fall of a prime minister at the brink of a military revolt.

Key points: A cabinet time capsule reveals concern from the Australian government over PNG's military acquisition

A cabinet time capsule reveals concern from the Australian government over PNG's military acquisition Russian-made military helicopters purchased by PNG were diverted to Australia due to security concerns

Russian-made military helicopters purchased by PNG were diverted to Australia due to security concerns The helicopters ended up in the Darwin tip

But the events that led to two Mi-24 gunship choppers being secretly buried in the hazardous waste section of the Shoal Bay landfill have largely been a mystery — until now.

Cabinet papers from 1998 and 1999, released by the National Archives, go some way to explain these curious events.

Australia's northern neighbour Papua New Guinea was having trouble quelling the separatist movement in Bougainville, which had developed into a full-blown conflict that eventually cost tens of thousands of lives.

Frustrated by failed attempts to have rebel leader Francis Ona attend peace talks, PNG Prime Minister Sir Julius Chan took the remarkable decision to contract British-based private military consultants Sandline International to help quash the secessionist movement.

The $50 million deal, signed in January 1997 to the horror of the then Howard government, would have seen foreign mercenaries flown in to destroy the Bougainville rebellion, using second-hand military equipment.

But two months later, on March 27, 1997, Australia agreed to a request from the PNG government to accept custody of the gear bought by PNG from Sandline.

"The PNG government was concerned about the delivery of the equipment to PNG in the uncertain political circumstance that prevailed at the time," then defence minister John Moore wrote in his confidential cabinet submission.

That was an understatement.

Sir Julius was facing angry public protests and a mutiny from his army. The Sandline affair precipitated the end of his prime ministership by June and the resignation of several ministers.

But Australia became the reluctant custodian of two Russian Mi-24 gunships built in 1983, armed with machine guns, plus two Mi-8 transport helicopters (built in 1972-73, cabinet was informed) and associated ordnance.

This included, Mr Moore told cabinet, ammunition, air-to-surface rockets, rocket fuse, signal cartridges and initiators.

It was quite the cache.

All of it had been delivered to RAAF Tindal, near Katherine, by a Sandline-chartered Antonov-124 aircraft.

The Russian-built Mil Mi-24 Hind attack helicopters arrived at RAAF Base Tindal in 1997. ( Supplied: ADF Serials Message Board )

But what to do with it?

Cabinet was advised that the machine guns and rocket pods attached to the helicopters were classified "weapons of war" and could only be stored on defence property.

But a defence inspection of the ordnance in December 1998 found that the rockets and fuses were the main concern. The rocket motors were "suffering significant cadmium corrosion", according to a cabinet note, making them a toxic hazard to anyone who came in contact with them. Their safety could not be guaranteed after December 1999.

On May 1, 1999, Bill Skate, who had replaced Sir Julius as PNG PM, announced he had reached an settlement with Sandline. PNG would pay the company $US13.3 million in four instalments over 12 months and all un-delivered equipment would be deemed the property of Sandline.

The then Australian foreign minister Alexander Downer was told PNG waived any ownership of the helicopters, the ordnance, the rocket pods, ammunition and everything else.

Cabinet wanted destruction of the ordnance but wanted Sandline to chip in half of the $32,000 cost, which was eventually agreed.

That left the not insignificant matter of four Russian helicopters that were taking up some valued space inside RAAF Tindal, a sensitive military base.

Cabinet considered charging Sandline for the inconvenience, on the hope that it "would increase the pressure on Sandline to find an acceptable buyer or to abandon the helicopters".

The cabinet papers reveal patience was running out with Sandline and its mercenary ways.

The helicopters deteriorated severely during their 20 years at the Tindal RAAF base. ( Supplied: ADF Serials Message Board )

"Notwithstanding the legal delays and some negative publicity, Sandline has done well out of its foray into PNG," Mr Moore told Cabinet.

"When the PNG government has paid its final instalment under the contract, Sandline will have received a total of $US31.3 million for its services.

"Considering the small amount of assistance and materiel that PNG received in return, it is likely that Sandline's profit on the deal was a healthy one."

He noted that the company's retention of the helicopters' ownership demonstrated Sandline's intention to further profit on the deal.

"Except possibly for spare parts, the helicopters are not a particularly good buy," the minister wrote.

But the whole episode had left cabinet with a sour taste.

"Sandline's controversial reputation, especially through its involvement in PNG and Sierra Leone, means that the Australian government's dealings with the company will be subjected to extra scrutiny," Mr Moore warned cabinet.

"It would be preferable not to publicly release the details of our discussions with Sandline until the issues are resolved."

In the end, a buyer for the Mi-24 attack helicopters could not be found.

City of Darwin's executive management of waste and capital works, Nik Kleine, at the tip used to dispose of the helicopters. ( ABC News: Jano Gibson )

The possibility of a new home at the Darwin Aviation Museum was abandoned when it was discovered the Soviet-era choppers were riddled with asbestos.

In July 2016, the helicopters were placed into a shipping container and quietly taken to the Darwin tip.

There they were buried. One last journey only recently uncovered.