David Lange rocks back and spreads his arms in anticipation. He is about to deliver arguably the most famous line in New Zealand politics. He looks as if he knows it.

It is the Oxford Union debate, 30 years ago this week, and New Zealand's prime minister is arguing for the motion that nuclear weapons are morally indefensible.

His young student opponent, speaking for a team led by United States evangelist Jerry Falwell, stands opposite on March 1, 1985, waving his finger as he challenges New Zealand to pull out of the Anzus alliance. "Whether you are snuggling up to the bomb, or living in the peaceful shadow of the bomb, New Zealand benefits, sir, and that's the question with which we charge you, and that's the question to which we would like an answer."

Rotund in his black bow tie and formal attire, Lange must have known the impact his next line would have. "And I'm going to give it to you, if you'll hold your breath for just a moment," he replied, the joy audible in his voice as the chamber erupted in laughter. "I can smell the uranium on it as you lean towards me."

And that was it.

He would say many more worthwhile things in his argument, but none grabbed the public with such force.

The line was actually Australian, as Gerald Hensley, head of the prime minister's department at the time, recalls.

Speaking this week from his home in Wairarapa, he remembered it was a few months after the 1984 Labour election win, but before New Zealand refused entry to the US warship Buchanan - because the US would not say whether it carried nuclear weapons - in early February 1985.

New Zealand had already made its intention clear to become nuclear-free. Hensley had been reading an Australian magazine and came across a cartoon of a person coming ashore on a Pacific Island. "I can smell the uranium on your breath," the local says.

Hensley thought it might amuse the prime minister. "I showed him it, and he did chuckle."

Nothing more was said of it until, a few months later, there it was on the world stage. In fact, Lange's opposition to nuclear weapons had been less than overwhelming. Before the 1984 election, he was reportedly assuring Washington it need not worry about an anti-nuclear policy.

Victoria University strategic studies professor Robert Ayson said, before the USS Buchanan incident, it is hard to gauge just how anti-nuclear Lange was, even though Labour campaigned on a policy of a nuclear-free New Zealand.

The public at the time was overwhelmingly anti-nuclear, but also wanted to keep the alliance with the US. It was soon clear we could not have it both ways.

The nuclear-free concept was sold as a New Zealand-only policy and there was no suggestion - to America's relief - that it would be exported.

That all changed in the Oxford university debating chamber.

Lange couldn't resist the temptation to up the ante and make it a "broader moral challenge" to a world going increasingly nuclear.

America would cut all military and security ties, effectively leaving the Anzus alliance as a pact in name only, but fears it would impact New Zealand-US trade never came to pass.

Technically, it would be two years before New Zealand passed legislation to become officially nuclear-free.

But, if the USS Buchanan incident left any doubts, New Zealand's stance was now out in the open - no more nukes would come to New Zealand, and nuclear-armed countries were morally corrupt. That was clear when Lange said rejecting nuclear weapons "is to allow a moral force to reign supreme". Shortly after, as the chamber rose to its feet for a standing ovation, there was no doubt he had won.

Grant Robertson - now the Wellington Central Labour MP - was a third-former at King's High School in Dunedin in 1985.

The teen was inspired by "an overweight guy with glasses" - an image Robertson could identify with - arguing on the world stage.

It was enough to propel Robertson into politics, even if he would not agree with all the policies - such as the sweeping "Rogernomics" economic reforms - that Lange's government would push through.

"But nothing will change the pride I felt about what he did at the Oxford Union."