Canberra students received the highest results, on average, in the latest nationwide literacy and numeracy tests.
However, they did not dominate the country's ranks of top-performers.
The Australian Curriculum, Assessment and Reporting Authority released aggregrate scores for each state and territory on Tuesday, from tests carried out earlier this year.
The ACT's average results were above the national average in each year group and discipline tested. Canberra also had the smallest share of children who failed to achieve a minimum standard in each skill: reading, writing, spelling, grammar and punctuation, and numeracy.
But despite Canberra's affluence and other socio-economic advantages, its students were not well represented in the highest-performing cohort for most areas tested.
The ACT's share of Australia's elite performers was highest only in year 7 reading and year 9 writing. NSW and Victoria tended to have a greater proportion of students in the highest-scoring categories.
Brawl over methodology
This year's results were published amid controversy over their validity, as one in five Australian students were tested online while the rest completed pen-and-paper exams.
The online NAPLAN test differs from the traditional version because it adapts to the child's performance, asking harder or easier questions to suit their ability, which allows a more precise assessment.
A paper by American education expert Professor Les Perelman, commissioned by the NSW teachers' union and released this week, argues the 2018 results should be ditched because there is no fair way to compare the online and paper tests.
However, the assessment authority's lead adviser, Melbourne University professor Ray Adams, rejected the union's paper late on Monday, saying its findings were untenable.
"It is my belief that Professor Perelman's report is under-researched and lacks the technical understanding required for the analysis of NAPLAN tests and results," Professor Adams said.
"Professor Perelman's expertise to make these claims is questionable as he is not recognised through the peer-reviewed academic literature as an expert in the field of computer adaptive assessment."
The ACT teacher's union joined calls on Tuesday to throw out the 2018 data, saying the results were corrupted and hence meaningless.
Branch secretary Glenn Fowler said NAPLAN comparing online exams with other tests reduced teachers' work "to a set of dodgy numbers".
"For ACARA to say that the polluting of 20 per cent of the 2018 results will not have an effect on the average is frankly laughable. In more than a decade, NAPLAN has given us nothing but perverse outcomes, controversy and dodgy data."
He said the testing regime lacked the support of the education community, including most teachers, parent groups and education academics.
Research highlights ACT 'underperformance'
The latest release of NAPLAN data coincided with the publication of a damning Australian National University report on underperforming Canberra schools.
The ANU researchers compared ACT schools with statistically similar schools elsewhere, and found "an alarming number ... where the [Canberra] students were, on average, more than six months behind the levels of learning of students in other comparable schools".
Their analysis of five years of NAPLAN results found evidence of widespread, systemic underperformance in the capital's public and private schools, across all socio-economic groups.
Canbera Liberals education spokeswoman Elizabeth Lee said the ACT government was failing the city's students.
"It is evident that there is only one reason why Education Minister Yvette Berry wants to do away with NAPLAN, and that's because it is a damning indictment of her own government's performance," she said on Monday.
"Instead of seeking to get to the heart of what we can do to achieve excellence in our schools, [she] has disgraced herself by campaigning on a national scale to discredit NAPLAN."
My School review
Ms Berry led calls earlier this year to review the My School website, which publishes schools' test outcomes. A national review is now under way.
She said she shared teachers' concerns that the reporting of NAPLAN results was a simplistic way to judge schools that "puts unfair stress on students, families and teachers".
This year's ACT budget provided $9.2 million for research and trials of new teaching techniques, which Labor committed to during the 2016 election campaign. The government has not yet released details of when the trials will start.