The presence of the Antonov meant that Sandline International, the military advisory company, and the company it subcontracts to, Executive Outcomes, had also arrived in PNG. To Mr Downer's horror "the world's largest private army" had pitched up less than 100 miles from Australia - with equipment, personnel and an array of sophisticated weaponry.
Its presence reflected a PNG vote of no confidence in Australia's ability to assist in resolving the nine-year secessionist conflict on the island of Bougainville by the Bougainville Revolutionary Army (BRA) which has consistently out-manoeuvred government troops.
This was all the more galling for Australia, since it has enjoyed a close relationship with its former colony, including a $12m (pounds 7.5m) defence contribution, military assistance and a $320m aid budget.
What led Sir Julius Chan, the Prime Minister of PNG, to privatise the training of his armed forces was his belief that his closest ally and friend, Australia, was frustrating his government's attempts to deal with the insurrection. "We ... have requested the Australians support us in providing the necessary specialist training and equipment ...They have consistently declined and therefore I had no choice but to go to the private sector," Sir Julius told The Independent.
His approach to the Bougainville problem in the early days of his administration was described by the Australians as "creative and courageous".
In 1994 he convinced the Australians to fund a short-term peace-keeping force drawn from South Pacific countries. He also initiated the Arawa Peace Conference in October that year, and in 1995 established the Bougainville Transitional Government. But all negotiations failed, due to a mixture of BRA intransigence and the ill-discipline of the PNG Defence Force, which made a habit of committing an atrocity just as a breakthrough was imminent.
Sir Julius was well aware of the lack of control he had over his army. With no assistance forthcoming from the Australians, he looked elsewhere and found it at 535 Kings Road, London, the representative offices of Sandline International. Notwithstanding all the reports about "guns for hire", the main task for Sandline and Executive Outcomes will be to whip the army into shape and to act as a "force multiplier" rather than as front-line troops.
Other, non-military, initiatives include buying back the Panguma mine on Bougainville, which is at the heart of the crisis, and directing funds towards economic redevelopment projects on the island.
The Papuans charge that the Australian approach to the problem lacks coherence. Canberra has been unequivocal in its robust condemnation of the BRA, and it endorsed and contributed to the PNG Defence White Paper in July 1996 which recommended "improving and modernising of existing capabilities in the PNGDF".
But Australia has repeatedly turned down requests for assistance in specialist training and procurement of sophisticated weaponry. Weapons and equipment were sold under the caveat that they were not to be armed and not to be used offensively in Bougainville. Australia also used its influence in the West to prevent other nations from supplying PNG with equipment. As well as being irritated by this paternal interference, the PNG government also felt that it smacked of hypocrisy.
The reasons for Canberra's reticence were simple. Despite training provided by the Australians, the PNGDF was not in good order. Offences and human rights violations committed on Bougainville acted as the best recruitment advertisement and propaganda weapon for the BRA. The Australians, wary of public opinion and of being associated with an army criticised by Amnesty International, sought to distance itself.
"What developed was a Catch 22 situation. The more the Australians distanced themselves, the worse it became. The worse it became, the more the Australians distanced themselves" says Sean Dorney, a correspondent with ABC who has lived in PNG for 11 years.
While the Australians accuse Sir Julius of employing mercenaries and of applying "a military solution" to a situation that will only backfire, Sir Julius continually reiterates his position that, whilst the national army remains a laughing stock there is little incentive for the rebels to enter into reasonable negotiation. He charges that Australia's "fence- sitting" has hamstrung the Defence Force and allowed a minority of extremists with no international justification to gain control of an area and impose their will on a terrorised people.
There are signs, however, that after the initial violent war of words over the affair, Australia and Papua New Guinea are coming closer. John Howard, the Australian Prime Minister, and Sir Julius held talks on Sunday in Sydney. Following the meeting, there were indications that Australia might increase its training of PNG soldiers. Mr Howard said: "We don't like mercenaries. We think any reasonable alternative to mercenaries is to be preferred."