Over the past 25 years, the number of high-income families across Australia choosing to send their children to private secondary schools has almost doubled, according to a new research report commissioned by the Australian Education Union.

Meanwhile, government secondary schools have almost doubled their share of low-income families compared with those from high-income backgrounds.

The report, by Canberra-based educational researcher Barbara Preston, also shows that despite historic links to supporting the education of poorer families, Catholic school families are now wealthier than government school families, although not as wealthy as the families who send their children to independent schools.

Using 2011 census data, Ms Preston said her report showed that government schools were becoming the residual system for low-income families, while high- and middle-income families increasingly sought places in Catholic and independent schools.

Using a low family weekly income of less than $1249, a medium family weekly income of $1250-$2499 and a high family weekly income of $2500, the report found that 42 per cent of students in government schools were from low-income families while 37 per cent were from middle-income families and 21 per cent were from high-income families.

Conversely, 23 per cent of independent school students were from low-income families, while 31 per cent were from middle-income families, and 46 per cent were from high-income families.

Catholic schools fell in between with 26 per cent from low-income families, 40 per cent from middle-income families and 34 per cent from high-income families.

The ACT was the only state or territory where there were more high-income families in government schools than low-income families. While Ms Preston cautioned that this was influenced by the general levels of wealth across the ACT, she also believed it was a result of community confidence in a high-performing government school system.

In the ACT, just 24 per cent of students at public schools were from low-income families, compared with 33 per cent from middle-income families and 43 per cent from high-income families. In ACT Catholic schools, 12 per cent of students were from low-income families, while 31 per cent were from middle-income families and 57 per cent were from high-income families.

In ACT independent schools 8 per cent of students were from low-income families while 22 per cent were from middle-income

families and 70 per cent were from high-income families.

Ms Preston acknowledged the ACT also had the highest proportion of students in the nation accessing private education - at about 43 per cent of enrolments. Yet she said her study showed the professional and managerial class in Canberra had faith in government schools.

Australian Education Union ACT secretary Glenn Fowler said the statistics were positive for Canberra but worrying elsewhere.

"Research says strongly that when children from higher-income families learn with students from lower-income families it has real benefits for the children from the lower-income families. And the benefits to children of mixing with all different types of children is great," he said. "Very clearly the ACT has high-quality government schools so we would ask parents why they would pay exorbitant fees charged by non-government schools - we'd say save the money and take the kids to Europe instead."

Nationally, however, the research findings painted a bleak picture.

"We are in real danger of having a two-tiered school system with residualised populations of low socio-economic students," he said.

AEU federal president Angelo Gavrielatos said: "This segregation is occurring at the same time as the resource gaps between public and private schools are growing, and government funding per student for private schools has been increasing at a far higher rate for a decade than it has for public schools."

Mr Gavrielatos also noted that while public schools were educating larger and larger proportions of students from low-income families - who had higher learning needs - the report found that student-teacher ratios had actually increased in public secondary schools since 1990.