The warning coincides with the release of a new report which shows students' literacy and numeracy skills have stagnated since the controversial NAPLAN tests were introduced in 2008.

"We need to be checking to make sure there is emphasis on not just reading to learn but learning to read as we get to higher year levels," ACARA general manager, assessment and reporting, Stanley Rabinowitz said.

He said the focus on reading instruction in primary school was paying off, but this needed to continue into high school.

"The assumption is that because we think they are reading, we don't have to do reading instruction in years 7 and 9," he said.

The 2015 NAPLAN national report, released on Wednesday, highlighted improvements in Year 3 reading, grammar and punctuation, and persuasive writing, and Year 5 spelling and numeracy.

The NAPLAN 2015 national report, released on Wednesday, highlighted improvements in year 3 reading, grammar and punctuation and persuasive writing skills and year 5 spelling and numeracy. There were also increases in Indigenous student reading performance in years 5 and 7.

There were also increases in Indigenous student reading performance in Years 5 and 7. The persuasive writing skills of year 7 and 9 students deteriorated.

Victorian students topped the country or were runners up in the five areas of testing in Years 3, 5 and 7. But in Year 9, this performance dropped off in spelling and numeracy, where they finished fourth in terms of the proportion of students who achieved or surpassed national minimum standards.

Victorian girls outperformed boys in all parts of the test, and across all year levels, while rural students lagged behind.

Australian Institute for Teaching and School Leadership chair John Hattie said many teachers stopped teaching the basics of reading in grade 2 because they assumed the majority of students had grasped the skills.

"Most schools don't have good reading programs in place for kids after they turn eight. Teachers create tasks for struggling students that don't require them to read, and they get further and further behind," he said.

Recognising and learning from outstanding teachers was the key to lifting students' literacy and numeracy skills, he said.

"At the moment we have a hierarchy based on experience rather than expertise."

ACARA chief executive Robert Randall said he had "hope(d) for more".

"Given the emphasis going into education in literacy and numeracy, we'd like to be seeing, in the long run, more improvement coming through."

The authority, which oversees the NAPLAN tests, flagged it would research why persuasive writing results were declining in high school.

There were 42 substantiated cases of cheating and breaches involving NAPLAN tests in 2015. Several of these incidents involved students sharing information about the tests on social media.

In Victoria, a staff member changed students' answers to the test, and was referred for disciplinary action.

A spokesman for Education Minister James Merlino said the government wanted to move Victoria's education system from "good to great". "We recognise that Victorian has a good education good system and we have maintained a high standard across all year levels in NAPLAN."