The school plans to open to female enrolments into Years 3 and 4 next year and into Years 7 and 11 from 2017, Principal Justin Garrick announced to staff, students and parents on Tuesday.

Describing it as a gentle transition, Dr Garrick said the aim was for all year groups to contain female students from 2020.

The decision had been a topic of discussion for a number of years but had received board approval just a few weeks ago, as well as the blessing of Bishop of Canberra and Goulburn Reverend Stuart Robinson.

Dr Garrick said he understood there would be some resistance from some parents who wanted their sons to attend an all-boy's school, but there had also been strong parental desire for the change to take place and Canberra Grammar needed to prepare its students to live and work in a world where women played an equal role.

"Becoming coeducational is an extension of normality," Dr Garrick said. "I think it is an important step to take for the benefit of our boys and also the girls for whom we want to open up our education facilities."

He did not believe there was conclusive evidence that girls or boys benefited purely from a single-sex environment.

"Every single student should be taught as an individual."

"We are also acutely aware that for many families that have sons and daughters, they deeply wish their children to be educated at one, excellent, school."

Dr Garrick noted "modern life is complicated enough without the requirement to place siblings of different ages and genders in multiple schools with different drop-offs and pick-up arrangements, different policies, procedures and communication systems, different dates, events and curriculum structures to negotiate."

The school would keep its name and introduce changes to the uniform which would be phased in from 2016.

Dr Garrick did not believe the school had an aggressively male culture and said every effort would be made to ensure new girls had "full citizenship" at the school from sporting, to cultural and academic pursuits.

The school would invest in new toilet and change rooms facilities in preparation, as well as taking expressions of interest in coeducational boarding from 2018,

With current enrolments at a record high of 1700, Dr Garrick said the move was not an attempt to double the student population. But extending enrolments to girls would "underpin the school's prosperity into the future".

"We face potential competition from the advent of new schools and from existing schools with the capacity to expand. It is virtually inevitable that the need to enlarge our enrolment pool through co-education will come upon as at some point. We believe it is right to embrace that reality now, on our terms, not later as a consequence of decisions made by others."

He conceded the move had the potential to impact Canberra Girls Grammar School, which is a constitutionally separate entity and which would be a direct competitor for enrolments.

"We have spoken with Canberra Girls Grammar about our decision and we hope to continue having a close and positive relationship with the school."

Fairfax Media is seeking comment from Canberra Girls Grammar.

The schools began a reciprocal enrolment arrangement over 40 years ago when they both allowed coeducational intakes from pre-school to Year 2.

Boys at the girls' school are guaranteed a place at Canberra Grammar at Year 3, while girls at the boys' school are guaranteed a place at Canberra Girls Grammar at Year 3.

Dr Garrick said it was always difficult for the school to lose its female students at the end of Year 2.

"It is often heartbreaking to lose our girls when they are in friendship groups with boys and happily ensconced in the Canberra Grammar community. They enjoy the nature of the place and many regret having to leave it."