The average ACT 15-year-old maths student is now one-and-a-half years behind where they were in 2003, according to the latest international test of student achievement.
The results of the OECD's latest Programme for International Student Assessment (PISA), released globally on Tuesday night, showed that while ACT students once again topped the nation in science, reading and maths literacy, their results are declining in real terms.
In scientific literacy ACT students are eight months behind where they were in 2006. In reading literacy they are one and a quarter years behind where they were in 2000.
Against the other states and territories the ACT is a year and a half ahead of the lowest performer, Tasmania, in scientific literacy; a year and a half ahead of the Northern Territory in reading; and a year ahead of Tasmania in mathematical literacy
The overall ACT results sit against national trends which show the same decline.
Australian students were the equivalent of one and a half years behind top-performer Singapore's students in science; a year behind them in reading and two and a third years behind in maths.
The trend data shows that an average 15 year old Australian student is now seven months behind where they were in 2006 in science, a year behind where they were in maths in 2003 and their reading ability has also declined by a year since 2000.
There are more Australian students in the lowest-performing cohort, and fewer high performers than in previous PISA tests.
In the ACT there has been a significant decline in the percentage of high performers. From 2003 to 2015 the percentage of high performers in mathematical literacy has dropped from 27 per cent to 14 per cent between 2003 and 2015.
ACT Education Minister Yvette Berry said she would "explore the information in these reports, together with other indicators, in pursuing opportunities for our schools and students to keep improving".
"The PISA report, alongside the TIMSS and NAPLAN, is showing us that while the ACT is significantly above the Australian average, we are experiencing similar trends in student performance as other jurisdictions across the testing periods.
"We have great schools and great teachers in the ACT."
She said she would speak to school leaders about the data and its usefulness for school improvement.
Federal Education Minister Simon Birmingham was a little more scathing.
"We must acknowledge the reality that our performance is slipping," Mr Birmingham said.
"Given the wealth of our nation and scale of our investment, we should expect to be a clear education leader, not risk becoming a laggard.
"Commonwealth funding for schools has increased by 50 per cent since 2003 while our results are going backwards. I'm not suggesting that money is not important, of course it is vital, but … Australia ranks as spending the fifth highest amount on education in the OECD and once you get to that level there is little value in just increasing spending."
Mr Birmingham is due to sit down with state and territory education ministers next week to hash out a deal on school funding.
In PISA Australia's results remain just above the OECD average overall, placing fourteenth in science, sixteenth in reading and twenty-fifth in mathematics, of 72 participating countries and economies.
Australia has now fallen behind Slovenia, New Zealand and Vietnam in scientific literacy, while it lags behind the Netherlands, Estonia and Poland in reading. Maths was our weakest area internationally, coming in behind countries including Sweden, Russia and Ireland.
The PISA results suggest the problem is even worse than revealed by last week's results from the 2015 Trends in International Mathematics and Science Study, which tested Year 4 and Year 8 students.
The TIMSS data suggested Australia was slipping against other countries but that performance levels were comparable with previous generations of Australian students.
But Dr Sue Thomson from the Australian Council of Education Research said the new PISA results show "Australian students' ability to apply their mathematical and scientific knowledge to real life situations is falling not only relative to other countries but also in an absolute sense," she said.
But as concerning as the international rankings are, Dr Thomson said, of most concern are the achievement gaps domestically, for indigenous and remote students and students of lower socio-economic status.
"The difference between advantaged and disadvantaged students is around three years of schooling. That's not changed in 16 years of testing (for reading). That's the critical thing. We're still not attending to those gaps."
Victoria is the only state to hold its position, where all other states and territories' performance has declined.
The goal of PISA is to measure how well 15-year-olds, who are nearing the end of their compulsory schooling in most participating educational systems, are prepared to use the knowledge and skills in particular areas to meet real-life opportunities and challenges.
The PISA 2015 test was completed between August and September last year by a representative sample of 14,500 Australian students from 750 schools. Globally 550,000 students participated. It is conducted every three years, administered by ACER and funded by the Australian government.
The focus of PISA 2015 was science. To access and complete some science questions click here.
With Kelsey Munro and Eryk Bagshaw