RISING fertility rates among older professional women and the power of My School data are driving the public schoolyard crush on Sydney's north shore.
The stranglehold of the families who settled on the north shore after the opening of the Harbour Bridge in 1932 is being released, setting off a demographic chain reaction that has led to the capacity crisis in its public schools.
All five comprehensive public high schools are full and many primary schools are at capacity with no more room for demountables.
The lower north shore experienced strong population growth after the opening of the Harbour Bridge in 1932, with families attracted by large land blocks and the suburban dream, putting down roots from the 1940s to 1960s.
Demographic change has meant many of those homeowners have become empty nesters who have moved out of their large family homes, leaving them available for new buyers who generally have children.
Dr Nicholas Parr, associate professor with the department of marketing and management at Macquarie University, said birth rates had spiked across the higher socio-economic areas of Sydney, including the north shore, eastern suburbs and parts of the inner west.
''Looking at the trends in the pre-school age groups, schools in these areas are not going to get any less crowded in the next decade,'' he said.
NSW Registry of Births Deaths and Marriages data shows Mosman, Lane Cove, Chatswood, Cremorne and Neutral Bay were among the state's top baby boom suburbs in 2011.
Figures from the Australian Bureau of Statistics show the number of children under five increased 54 per cent between 2001 and 2011 in North Sydney, and in Willoughby they increased by 32 per cent.
Emeritus Professor Ian Burnley, of the City Futures Centre at UNSW, said women in their 30s were leading the charge in the fertility stakes and educated women tended to put off child birth until they were older.
''Women from high socio-economic backgrounds are more likely to have babies later in life,'' he said.
''They go to university, they travel, they establish careers and they find partners later and have children when they are older.
''The north shore is a higher socio-economic area of Sydney so that's where these families tend to settle.They also settle in the eastern suburbs and inner west but housing prices on the north shore are less expensive relative to the east and inner west and the houses tend to be bigger. So, if you have a family, those houses have obvious appeal.''
Data available on the My School website has driven families to settle also in the catchment areas for the top public comprehensive schools such as Killara High, Willoughby Girls, Chatswood High, Mosman High and Killarney Heights High.
The Northern Sydney Regional Council of Parents and Citizens Associations executive member Steph Crofts said parents kept a close eye on the academic performance of the public schools. ''You hear of families moving into the catchment areas for those highly ranked schools to get their kids in,'' she said. ''I think that explains a lot of the movement.''