As more ACT schools suffer an enrolment crush, the Education Directorate has changed the definition of school capacity – allowing it to add hundreds of "new" places to schools that were previously quarantined for special education and non-mainstream classes.

Opposition education spokesman Steve Doszpot said it was an "outrageous manipulation of data" that allowed the government to evade criticism for letting schools enrol more students than their mainstream capacity.

The Australian Education Union said it would seek a briefing from Education Minister Shane Rattenbury while the ACT Council of Parents and Citizens Associations said changing the definition "appears to be a very cynical move".

Late last week, Mr Rattenbury released enrolment projections for Canberra's public schools to 2018. He had earlier announced the government would spend $20 million to provide an extra 680 student places in Gungahlin schools to ease enrolment pressure there.

But the latest official data shows a boost to most schools' capacity, which happened simply by changing the way capacity was calculated.

Capacity is defined as "the total number of students that a school can hold before any additional infrastructure works is required. This capacity figure includes all learning and teaching spaces including curriculum flexibility spaces and special education spaces, based on a maximum number of students able to be supported in these centres and units."

At least 74 learning-support centres and units, and autism-support units, operate across the ACT's public schools. These contain six to eight students each but were historically excluded from capacity figures.

The previous 2013 Act Public School Enrolment Projections calculated capacity for "mainstream students only". It said: "Analysis of the capacity of individual schools takes into consideration a range of design factors and specific education needs", which includes "the operation of special programs in schools, e.g. bilingual immersion provision".

The latest data shows some schools increased their capacity by tens, if not hundreds, of students between the 2013 and 2016, despite gaining no major infrastructure, classrooms, demountables or investment.

Mr Rattenbury said the new definition was more accurate as "the traditionally reported definition of capacity (total capacity) was an internal definition that did not completely match the enrolment definition, as it excluded the capacity of specialist units accommodated at a school, such as learning-support units and introductory English centres, even though the students that used these units were counted in the enrolment figures".

"The new definition is designed to match the enrolment definition and show both how many students are enrolled and how much room there is for those students. The change has not added any new places across Canberra schools, as these places always existed," he said.

But Mr Doszpot said adding in previously quarantined students made it look like schools had more room than they once had.

"I have been trying to call this government account on enrolment pressures for two years. And all of a sudden the definition is changed," he said.

"I can tell you, there is nothing wrong with the previous definition, but there is something very wrong with schools increasing their sizes without adequate planning."

He accused the government of "wilful blindness in not recognising the impact on the community at large of urban densification", saying schools such as Harrison and Garran were prime examples of areas where increasing numbers of families were living in units and adding pressure to in-area enrolments.

Australian Education Union ACT branch secretary Glenn Fowler said he would discuss the new definition with the minister but said the union would be concerned if it had any further effect on mainstream class sizes.

"We are already well aware of some schools coming under significant enrolment pressure. We are not interested in class-size averages, but are interested in ensuring no teacher is forced to teach a class above maximum size," Mr Fowler said.

"We know there are now some flashpoints in the system and this risks teachers facing the risk of burnout but also will have a significant impact on student learning."

The P&C Council said parents had a right to be disappointed if their child was taught in a space that was not designated for teaching – such as a staffroom or library – which is commonplace in some overcrowded schools.

"Changing the definition of capacity seems to be a cynical move. You can't just define this problem away," council president John Haydon said.

The council executive met this week to discuss capacity, with Mr Haydon saying: "There is a strong feeling that these are genuine problems that are not being dealt with adequately."

"It is hard to understand in a city such as Canberra that is so largely planned, that it is so hard to get the planning of school provision right," he said.