Australia's top-performing primary school students are reading better than five years ago but the poorest readers haven't improved.
Overall, Australia has jumped to 21st from 27th in the most recent international literacy test of Year 4 students, released late on Tuesday.
It was one of 18 countries with significantly improved results in the 2016 Progress in International Reading Literacy Study.
Four in five Australian students met standards for what is considered a proficient reader.
But Australian Council for Educational Research deputy chief executive Sue Thomson says while it's good there are more students now at high or advanced levels than in 2011, it is concerning there aren't fewer poor performers.
About one in 15 Australia students didn't reach the lowest benchmark, meaning they couldn't relay information from simple texts or make straightforward deductions about events and actions mentioned.
Another one in eight scored at this lowest level.
Dr Thomson suspects the last batch of international reading test results from 2011 and 2012, which showed the number of top performers shrinking, shocked educators into concentrating more on those high achievers.
"But there's still nothing particularly has been done to address low levels of literacy," she told AAP.
"To improve the performance at the lower levels is a much harder job. It's something that requires a really concerted policy effort to be able to turn it around."
Education Minister Simon Birmingham said the results were encouraging.
"But it's also clear there's no room for complacency," he said.
"Australian educators and policymakers must keep focused on what needs to be done to further boost student outcomes."
Dr Thomson also examined factors that affect students' performance.
Many of these are outside a teacher's control, like students coming to school hungry, tired or not at all, the resources available, and how highly a school values academic achievement.
She says the persistent gap in performance between students at disadvantaged schools and affluent ones isn't just because the school is poorer but because all of these factors are more likely to be present.
Similarly, richer schools were more likely to have more students starting their education with reading skills.
In countries that outperform Australia, like Ireland, nearly all students attend a school where at least three-quarters start out being able to read.
In Australia, just six per cent attend such schools.
The majority of Australian primary students were at schools where less than a quarter of children started with reading skills.