THE big decline in the proportion of Australians getting married imperils the recovery of the fertility rate, a new analysis of 2006 Census data shows.

Falling marriage rates at every age threaten the sustainability of the much-heralded mini baby boom because overwhelmingly it is married women who have the nation's babies, says Bob Birrell, director of the Centre for Population and Urban Research at Monash University.

The analysis, prepared for the Herald by Dr Birrell and Genevieve Heard, a research fellow at the centre, shows that despite hopes raised by the recent rise in births, the trends point to a long-term decline in the nation's fertility rate.

By 2006 women at each age group had had on average fewer babies than women in 1986 or 1996, the data shows. And the proportion of women at every age with no children has continued to rise over the past two decades.

"Talk of the mini baby boom may seem optimistic in light of this data," Ms Heard said.

The explanation for the long-term decline may lie in the strong association between marriage and babies despite the social acceptability of cohabitation.

"Marriage is still so important to sustaining a relatively stable fertility rate," Dr Birrell said. "Cohabiting doesn't serve the same purpose in terms of childbearing."

The latest census shows the steady decline in marriage rates over the decade to the point where less than 50 per cent of Australians over 15 were married last year. The number of people who had never been married increased by 24 per cent over the decade or by about 1 million people.

The story of the relentless decline in marriage rates is the same for every age group. At age 30-34, for example, 49 per cent of men were married compared with 57 per cent 10 years earlier. At the same age, 56 per cent of women were married compared with 65 per cent 10 years earlier.

But the decline in marriage rates did not reflect a preference for the single life, Dr Birrell said. A growing proportion of couples had elected to cohabit, arresting a 15-year decline in overall partnering rates (marriage and cohabitation) that was evident until five years ago.

Among men aged 30-34, the proportion cohabiting had risen to 18 per cent from 11 per cent a decade ago and among women from 9 per cent to 15 per cent.

The general reluctance to have children in a cohabiting relationship was demonstrated by the wide gap in proportions of childless wives and childless de facto partners, Dr Birrell said. At age 30-34, for example, 47 per cent of female de facto partners have no children, compared with 21 per cent of wives, and at age 40-44 only 8 per cent of wives are childless, but almost one-quarter of partners do not have children.

"The decision to have children reflects a couple's sense of security in each other and in their future economic wellbeing," Dr Birrell said. "Marriage is a symbol of this. What matters is to create a society where people feel secure about their future."

Nicky French, 35, of Bardwell Valley, lived with her husband Cameron for nearly four years before they married, and nine months to the day after their wedding Oscar, now four, was born. Matilda-Lily came two years later.

"Marriage feels more like a commitment, something I would not walk away from without trying very hard," Mrs French said, "and I wanted that before I brought children into the world."

Peter McDonald, Professor of Demography at ANU, said the declining marriage rate "was not a big crisis". People were delaying marriage, but between 75 and 80 per cent of Australians would still tie the knot "and as long as those who get married continued to have their two children the fertility rate will be sustained" at 1.8 births per woman.

That was sufficient, with continuing migrant intake, to forestall a crisis over the country's future labour supply, he said.