The officials working late in the Beehive on that July evening in 1985 must have registered some grim satisfaction as they compiled the dossier of incriminating documents that would be dispatched to Paris later that night.

While the New Zealand public was still unaware of just who was behind the bombing of the Rainbow Warrior, Cabinet ministers had known within three days of the blasts that the finger of suspicion was already pointing firmly in France's direction.

But their disbelief and anger alone was not going to breach the wall of denials, lies and obstruction being hastily erected in the French capital.

A week later, the New Zealand Government had gathered sufficient evidence to bring down the curtain on this Gallic farce.

The diplomatic bag containing the confidential police file detailing progress of the 10-day-old investigation into the sinking of the Greenpeace vessel was forwarded to the New Zealand Embassy in Paris and delivered to the ornate headquarters of France's foreign ministry on the Quai d'Orsay.

The ministry's diplomats - the elite of the sprawling French bureaucracy - were to get a rude shock. But they quickly realised the game was over, even if other arms of the French Government would prove more reluctant to embrace the embarrassing truth.

The New Zealand Government was effectively saying: "We know enough to know France did it. Here, judge the evidence for yourself."

After the Rainbow Warrior was sunk in the Waitemata Harbour on the night of July 10, New Zealand authorities initially treated the sabotage as a purely criminal matter, rather than the assumed work of terrorists.

To those suspecting the French were the likely culprits given Greenpeace's campaign against testing of its nuclear force de frappe on Muroroa Atoll, Paris had issued a very loud "non".

The outright denial was something of a relief to New Zealanders unwilling to contemplate that a friendly country could cold-bloodedly sanction what David Lange would later describe as "a sordid act of international state-backed terrorism" and what Sir Geoffrey Palmer still calls "an act of war".

However, with two suspects in custody, the New Zealand police and the Security Intelligence Service tracking the movements of some 13 French agents, knew otherwise.

Twenty years on, Gerald Hensley, who was head of the Prime Minister's Department, has told the Weekend Herald of the Lange-approved plan to confront the French with the evidence.

"When we initially said we knew all about the bombers, they just laughed," he recalls. "It was only when we produced the file that they realised just what detail we had of the agents' movements. They acknowledged their secret service had been in New Zealand. They started talking to us on more realistic terms."

It would still be two months before that realism, assisted by leaks to the French media, would win over attempted whitewash and finally bring a humiliating admission from the Fabius government that the DGSE, the French secret service, had placed the explosives that sank the Rainbow Warrior.

The overnight announcement from Paris was the cue for an early morning prime ministerial press conference in Wellington, which saw an ebullient Lange performing a delayed act of national catharsis as he vented the deep public fury at France for its blatant breach of international law.

"It was the end of innocence," declares Sir Geoffrey of the unprecedented violation of New Zealand sovereignty.

While the-then Deputy Prime Minister doubts there was much spin-off for Labour, others say the humbling of France did an already-popular Government no harm.

"There was a sense of patriotism and a government always wins out of that," remembers Richard Prebble, who was also a Cabinet minister. "Lange, who had a great sense of polls, rode it for all he was worth."

Sir Geoffrey agrees. "Lange ran this from beginning to end and he ran it very well. He was always inspired in a crisis. He was decisive ... and he was outraged."

The two former ministers also believe the bombing - in the immediate aftermath of the Anzus rift - helped cement Labour's anti-nuclear policy into an icon of national identity. A nuclear power was behaving like the neighbourhood bully. New Zealanders were indignant, but proud that French arrogance and duplicity had been exposed.

However, France proved impervious to shame. Soon enough, Paris was applying what Sir Geoffrey calls "relentless and unreasonable pressure" to secure the release of Alain Mafart and Dominique Prieur, the two agents caught by police before they could fly out of New Zealand.

Sir Geoffrey says that as early as September 1985 - barely two months after the bombing - he agreed to talk to French Foreign Minister Roland Dumas while the pair were in New York on United Nations business.

The two meetings at the New Zealand mission - the precursor to secret negotiations the following year - amounted to a French fishing expedition to test the New Zealand Government's resolve.

While the French argued the pair were guilty only of following orders, Sir Geoffrey insisted New Zealand justice would run its course. It did. Mafart and Prieur were convicted of manslaughter and sentenced to 10 years' jail in December 1985.

Lange point blank refused to deport them. Paris retaliated by blocking New Zealand exports, notably lambs' brains, from entering France.

"This was a very, very unprincipled act to try to coerce us," says Sir Geoffrey of the big power-small nation imbalance.

But he and Lange could see the much bigger writing on the wall - a French veto blocking New Zealand's vital access to the British lamb and butter market, which was up for renegotiation in Brussels.

"It was clear New Zealand could not win a trade war against France."

In June 1986 Lange formally announced the dispute would be mediated by United Nations Secretary-General Javier Perez de Cuellar. In fact, New Zealand officials, led by the late Chris Beeby, had already been secretly hammering out a deal with French counterparts in Geneva, but the secretary-general's imprimatur was required for the settlement to carry weight.

New Zealand got a formal apology, trade undertakings and $13 million in compensation. France secured the release of Mafart and Prieur from New Zealand prison to the French military base on the barren Pacific atoll of Hao, where they were to be confined for three years.

Sir Geoffrey argues the terms of the settlement show that diplomacy worked in New Zealand's best interests, especially in explicitly stopping France from destroying New Zealand's butter trade with Britain.

"We could not have really got a better outcome. Principle was vindicated. The French cynically thought we would have to comply with what they wanted. But we held them to account in international law much more effectively than they expected we could."

Prebble sees it differently. He and other Cabinet colleagues were resigned to the inevitability of a deal following the agents' convictions.

But striking one was made even more difficult by France's failure to put up a defence for Mafart and Prieur at their High Court trial. Given they were first offenders and accessories to manslaughter, Prebble argues they could have expected relatively light sentences. Instead, they got 10 years - terms which Prebble says delighted the public, but horrified the Cabinet.

"It was obvious the agents could not serve that long. The truth of the matter was that both governments were keen to move on and the UN settlement's purpose was to placate public opinion."

It did not.

Lange had always coupled his trenchant criticism of the French Government with the proviso that he did not want to jeopardise relations with Paris. He had also drawn a distinction between releasing the two agents to freedom and releasing them to French custody.

However, his frequent declarations the New Zealand justice system was "not for sale" were thrown back at him by critics when the settlement was announced.

Hensley says officials had urged Lange to be more cautious in his statements, while Prebble says the Cabinet was worried by the strident tone of his language.

Prebble believes Lange should have softened up the public to the necessity of resolving the dispute as soon as possible, given the importance of the bilateral relationship. Sir Geoffrey disagrees, saying Lange could hardly have been tough one moment and weak the next.

No one was surprised when the French later breached the settlement and repatriated the agents to France on dubious medical grounds, firstly Mafart in December 1987 and Prieur the following May.

New Zealand, powerless to stop them, invoked the settlement's arbitration procedures.

In May 1990, an international panel determined the agents not be ordered back to Hao, but recommended France contribute $3.5 million to a "friendship fund" to foster better ties with New Zealand.

A year later, French Prime Minister Michel Rocard delivered a final, more sincere apology on New Zealand soil at a state luncheon in the Beehive banquet hall, just seven floors below where those officials had worked so assiduously on that July evening six years earlier.

* The Weekend Herald sought to interview David Lange, but he declined.