Data from the Education Directorate's School Census show that enrolments have declined 25 per cent at St Edmund's College, 21 per cent at St Clare's College, 21.6 per cent at Merici College and 2 per cent at Marist College over the past decade.

Canberra Girls Grammar enrolments rose 10 per cent over the same period. Over the past year, Merici recorded the biggest decline in enrolment, down 9 per cent, while Marist recorded no change at all.

Canberra Grammar, which started the transition to providing tuition to all students in 2015, recorded a 22 per cent rise in enrolments over 10 years and a 9 per cent increase on last year.

Merici College principal Loretta Wholley has taught in single-sex schools for about 12 years and attended a single-sex high school.

She said she believed "first and foremost" in the academic advantages of gender segregated schooling which made girls feel more comfortable in pursuing traditionally male subjects and vice versa.

"Especially in the maths, science, PE, IT area, those areas that can be dominated by boys in a co-ed school, there's no such thing here," Mrs Wholley said.

"Every girl has the opportunity to be a leader, so whether it's the head of the volleyball team or the head trombone in the orchestra or part of a school play or the leadership, all of those kinds of things, they're really all given to girls."

Merici College year 8 student Lucy Pembroke, year 10 Jade Esler, and year 12 Maddison Berry said they all made the switch to a single sex environment because they felt it better catered to their needs.

"At single-sex schools there is no such thing as a male subject or a female. Every subject can be a subject for you," Lucy said.

"It's not like we don't have any contact with boys, I see plenty of my friends from primary school who are boys on the weekends ... and it's good to be in a classroom with just girls because you don't feel that pressure to kind of impress and not be too in the spotlight all the time."

St Clare's College principal Brad Cooney and Mrs Wholley pointed to new Catholic secondary schools and changing demographics as contributors to a decline in enrolments. Both schools were on target for enrolments for 2018, they said.

Marist principal Richard Sidorko said his all-boys school had fared well because of its focus on providing a quality all-round education.

"I think while we continue to make the offerings that we do and produce the outcomes that we do, I think there'll be a place for a single-sex boys' school in Canberra," he said.

A study published in Science in 2011 found evidence to suggest sex-segregated schools increased gender stereotyping and gave legitimacy to institutionalised sexism.

The Pseudoscience of Single-Sex Schooling, led by former American Psychological Association president Diane Halpern, found there was little evidence that gender segregation produced better educational outcomes once results were controlled for other differences.

As well, "Boys who spend more time with boys become increasingly aggressive ... girls who spend more time with other girls become more sex-typed", the report found.

But the Alliance of Girls Schools of Australia points to Australian research which found "inherently different schools", including single-sex schools, did better when it came to tertiary entrance rankings.

And the 2016 ACT Catholic Secondary Education Survey found little appetite among parents for a further shift to co-educational schools.

"Female students in years 7, 11 and 12 showed higher than average support for single sex structure or single-sex class options within the co-ed school," the Archdiocese of Canberra and Goulburn Catholic Education Office.

"Parents and caregivers expressed higher than average support for single-sex female structure. There is a higher than average support from parent/caregivers (pre-secondary) for a single-sex male structure."

Other single-sex principals were unable to be contacted. Also over the decade, enrolments at St Francis Xavier College declined 0.3 per cent, 0.7 per cent at Daramalan College and doubled at Burgmann College.

St John-Paul II College, which opened in 2013, had 636 students in February this year, while enrolments at St Mary MacKillop College have increased 13.5 per cent since 2007.