Former New Zealand Prime Minister Helen Clark says it's extraordinary how few women are in positions of power around the world.

"Today, there are only eight women heads of state - representing slightly more than five per cent of the total," Clark, the United Nations Development Programme head, said in a key note speech in Wales overnight.

"This seems extraordinary in the second decade of the 21st century."

She spoke about her nine years as prime minister, saying the wealth gap between this country and other richer countries was largely due low levels of labour force participation of New Zealand women here.

"There can be no real democracy if half the population is excluded from participation and power," Clark said.

"Athenian democracy, where only some men had voice, will not do in the 21st century."

She said the first woman who was not a monarch to become a national leader was the Prime Minister of Sri Lanka, Sirimavo Bandaranaike, in 1960.

There were three female leaders in the 1960s, six in the 1970s, and seven in the 1980s.

"The total numbers of women who have reached these positions have risen only slowly since, with New Zealand supplying two."

Clark said the global average of women holding parliamentary seats remains under 20 per cent.

"There are currently only 41 women heads of parliament. Only sixteen per cent of ministers are women, and most often they are allocated portfolios like those for social welfare, women, and children."

The proportions of women in national legislatures in the world's regions range from roughly 22 per cent in the Americas and Europe (with the 42 per cent in Nordic countries pushing the average figures up) to 20.2 per cent in sub-Saharan Africa, 17.9 per cent in Asia, 14.9 per cent in the Pacific, and 10.7 per cent in the Arab States.

Five countries - all in the Gulf and the Pacific - have no women parliamentarians at all.

Clark said women with equal rights, as basic as being able to own and inherit land and property, access credit, and open bank accounts, can play an even greater role in the development of society.

"The gap between New Zealand's GDP per capita and that of Scandinavian countries owes a great deal to the lower level of labour force participation of women in New Zealand.

"That was one of the reasons why my government pursued work-life balance policies, like implementing the extra week of annual holidays and a right to paid parental leave.

"The universal right to twenty hours free early childhood education was established both because of its importance for children and because it made the option of paid work for both parents a realistic one," Clark said.

If women were out of sight and out of mind, their needs did not get priority, she said.

"Conversely, when there is a critical mass of women decision-makers, the issues which previously went unaddressed can become priorities."

Clark praised Rwanda for having the highest proportion of women parliamentarians in the world - currently at 56 per cent.

"It is no surprise therefore that its parliament drafted a far-reaching law to combat gender-based violence."

Clark said she took great pride in having led a government in New Zealand which did look at policy through a gender lens, and implemented many policies of significant benefit to women.

"Examples include enshrining in law the right to paid parental leave, and to a statutory minimum entitlement for all to a fourth week of annual holidays. Twenty hours free early childhood care and education and interest-free loans for tertiary students were also of particular benefit to women."