Ms Clark, who is on course to win Saturday's general election, refused to say if she had ever even placed a joint in her mouth when interrogated about her experience of the drug during a televised debate last night with her adversary, the Prime Minister, Jenny Shipley.
"Do you want to bring politics to this level?" Ms Clark asked the interviewer, Paul Holmes. "Look, I attended university in the late 1960s. It would be foolish for any politician or indeed person around circles in those areas to deny that they ever saw it." Mr Holmes joked: "Did you ever lie on the floor and listen to Peter Frampton with headphones on?" "No, I didn't," she replied.
Marijuana has become a burning issue in an otherwise dull campaign, because the Green Party, which looks likely to win several seats, advocates decriminalisation. Labour is committed to reviewing the law, and Ms Clark, 49, said last night that she supported "partial decriminalisation", with personal possession punishable by a fine.
Mrs Shipley, 47, was a student in a similar era, attending teacher training college. But she stated categorically on national television that she had never smoked the dreaded weed. "The use of cannabis leads to the use of other drugs, and National opposes that."
If daggers were drawn during the debate, they were kept well out of sight. The two women competing to run New Zealand were the soul of politeness during the hour-long encounter, which revolved mainly around their plans for the economy.
Ms Clark, who plans to increase taxes for high earners, criticised National's pledge to reduce personal tax. "What we know is that taxes too low mean we can't fund the schools properly, we can't fund tertiary education properly, we can't fund the health system properly, we can't give a decent pension."
Mrs Shipley, whose party has been in government for nine years, retorted that Labour's tax increase "won't bring jobs, it won't bring prosperity, it will simply shrink our economy".
Ms Clark gave a rare insight into her personal life when she was asked about her decision not to have children and whether it affected her ability to understand the pressures faced by families. "I haven't had children and that may be a source of enormous sadness to me one day. But I come from a family of four girls. I'm the oldest. I have eight nieces and nephews, my husband has seven."
Both women hope to become New Zealand's first elected prime minister. Mrs Shipley got the job two years ago after mounting a coup against Jim Bolger while he was at a Commonwealth heads of government conference in Edinburgh.
In pitches at the end of the programme Ms Clark urged New Zealanders to vote for her caring brand of social democracy, promising a new, fairer society "which offers all our people opportunity and security". Mrs Shipley pointed to positive forecasts for the economy. "Why would you change a successful formula?"
Ms Clark was considered by political commentators last night to have triumphed in the televised debate. "A decisive win on points to Helen Clark," said John Roughan, assistant editor of the New Zealand Herald newspaper.Reuse content