There is a curious lack of interest in Hong Kong over the issue of how far and fast its population should grow and indeed in critical issues raised by its current and future demographics.

Last week the government Census and Statistics Department published its population projections for 2010-2039, data which reach into many areas of government policy. But it went largely ignored by the media.

It is not as if population growth is something outside the government's remit. Furthermore, some past over-estimates in government long-term planning documents have been so large that there have been suspicions that they have been pumped up to justify all kinds of infrastructure projects designed to keep influential contractors and government departments busy.

Using a 2009 baseline of 7.003 million the forecast is for the total to reach 8,892 million by 2039, an annual increase of some 0.8 percent or 63,000 a year. This may seem a surprising increase given that Hong Kong has one of the lowest fertility rates in the world – currently 1.042 births per women of fertile age – and is projected to fall further. The territory also already has a median age of 40.7 years and this will rise to 47.6 by 2039.(The figures would be 41.4 and 49.0 if foreign domestic helpers, who have no residence rights, were excluded from the population).

Some of the projected increase in population can be accounted for by assumed rising life expectations. At present this is 79.8 years for men and 86.1 for women and is projected to increase gradually to 83.7 for men and 90.1 for women by 2039. This may itself be an over-optimistic assumption given the impacts of pollution and obesity, both relatively new phenomena, on those now in middle age. But it is a slower gain than the 5.5 year rise in life expectancy achieved since 1989. In any case it illustrates just what a huge adjustment Hong Kong faces over the next 30 years in dealing with a much older population. Now only 6.2 percent of the population is 75 or more. That will increase to 16 percent by 2039. At that time only 10.5 percent will be under 15 years of age.

The ageing process would be even faster were it not for assumptions of population inflow that could prove to be over-estimates. They are also the ones more subject to government action. The first is the number of one-way permit holders arriving from the mainland – those permitted to remain as permanent residents. This has been steady at 150 a day or 54,000 a year since 1995 and is assumed to remain the same though there is potential to alter it. There are complaints in Hong Kong that it is not only too many but that most are unskilled and add to social problems due to the shortage of unskilled work and remote locations of much low cost housing. Both the level and choice of one way permit issuance should be a matter for public discussion – but never is.

The second issue is the number of babies born in Hong Kong to mainland parents with no right to live in the territory. Their babies have that right so the issue is what percentage of them eventually return to live in the territory. The government, estimates, using surveys of parents, that though almost all babies return to the mainland after birth 62 percent are expected to return to Hong Kong eventually. But this is highly speculative and begs many questions about relative living standards in Hong Kong and nearby China in 15 to 20 years.

It is also important. In 2009, of the 82,095 live births in Hong Kong, 37,253 were to mainland women, of which 29,766 did not have a spouse with permanent residence.

The number of mainland women giving birth in the city is in turn subject to government influence – either by not allowing women in late pregnancy to visit or by further increasing fees charged to them in public hospitals.

The schooling and abode decisions of the parents and the babies, when they grow up, have major implications for Hong Kong's school system and for its supply of young labor in the future. The government's projections are useful but are plain guesswork. If numbers returning are significantly less, the total population may well barely increase at all – particularly if the numbers of permanent residents who are mobile and actually spend more time on the mainland or in countries such as Canada continue to increase. At present of the 7.0 million population, 206,000 are characterized as "mobile residents" – ie people not here most of the time.

The third category which will significantly influence both the total and the age distribution is the number of foreign domestic helpers. These now number 265,000 and are forecast to increase to 389,000 by 2039 – a rate of growth far in excess of the increase in the number of households. In turn that depends on government policies towards such foreigners, including their wage levels.

The numbers of domestic helpers will make Hong Kong very odd compared with the rest of China where men will greatly outnumber women in the young and middle age groups. In Hong Kong there will be, as now, an approximate 10 percent excess of males in the 0-19 age group but in all other age groups there will be a big excess of women, with women overall being 57 percent of the population by 2039 compared with 52 percent today. In the 25-39 group there will be 1.04 million women for 0.66 million men. Of course these are just projections. But they are projections from current trends and seem likely to be at least partly fulfilled – depending on government policies.

Indeed, that applies to all these projections. But they should be a point of departure for discussion of a wide range of policies, on immigration, on housing, on care for the aged, on the desirability of increasing reliance on foreign domestic helpers, on priorities in public investment, etc.