The Australian Curriculum, Assessment and Reporting Authority has issued the preliminary findings of 2015 NAPLAN results in reading, persuasive writing, spelling, grammar and punctuation, and numeracy across the nation's Years 3, 5, 7 and 9 students.

The ACT came top or equal top in all domains except for Year 3 writing, where it came second behind Victoria and Year 3 spelling, where it came third behind Victoria and NSW.

The ACT's student withdrawal rate had also dropped across all year levels compared with 2014. This year, the Year 3 withdrawal rate was the highest in the nation at 4.7 per cent. But it was down on the 2014 rate of 4.9 per cent.

ACT Education Minister Joy Burch said the 2015 data "confirms ACT students continue to be among the highest performing across the country".

"ACT students continue to excel in reading and have been the highest performing jurisdiction in Australia every year since 2008."

Last year the ACT came top or equal top in 16 of the 20 domains, which was down from 17 in 2013.

ACT results this year consistently outrank the national average in all 20 domains, and it has improved its overall rankings slightly on last year.

Nationally, the picture is similar with ACARA reporting that "while the results show some improvements nationally and in each state and territory, for some year levels and some domains, the overall message is one of stable national achievement", according to chief executive Robert Randall.

"We are seeing little change in student achievement in these important areas of learning. While stability is good, the community may well expect more improvement over time."

Notable national improvements since the NAPLAN testing regime came into place in 2008 include a significant improvement in Year 3 reading – a continuation of the long-term trend.

Numeracy at Year 5 saw a statistically significant increase from 2008 as did Year 5 spelling.

"While the ACT, NSW and Victoria continue to achieve high results the 2015 results show that Queensland and Western Australia have made significant gains in several domains, and their efforts should be applauded," Mr Randall said.

Ms Burch said the ACT had ranked first or equal first in grammar and punctuation, and numeracy across all year levels since 2009.

"These outstanding results are due to the fantastic work put in by our teachers, and for that I'd like to thank them."

The NAPLAN 2015 summary information includes data at the jurisdiction level and provided raw mean scores which the ACT Education Directorate said could not be compared directly across jurisdictions.

The national report, which will include results broken down by categories, is scheduled for release in December.

About 18,600 students sit the tests across the ACT with individual student reports to be released to schools for distribution to parents from Tuesday, August 18.