Testing was suddenly halted on Wednesday morning after students and teachers were unable to access the writing component of the annual assessment online.
The Australian Curriculum and Assessment Reporting Authority (ACARA) confirmed the outage was widespread affecting students trying to log in to the online test portal.
Grade 3 students were not impacted, as their assessment is completed by hand.
"This issue is being urgently investigated by our technology provider, Education Services Australia, who run the platform," ACARA said in a statement.
More than 1.3 million students in grades 3, 5, 7 and 9 were set to begin testing on Wednesday, although the full extent of the outage is not yet clear.
An authority spokesperson confirmed testing had resumed after 12pm on Wednesday after issues relating to the online portal were fixed.
"We apologise for the disruption to students and schools, and thank them for their patience. The issue has now been resolved, and schools have been informed they can resume testing," the spokesperson said.
'We continue to monitor the platform to ensure students are able to complete their assessments without further issues."
Educators took to social media to describe the confusion and interruptions following the widespread outage.
A Victorian secondary school teacher told AAP the disruption was difficult to manage, with students left nervous and unsettled by the outage.
They hope NAPLAN would return to pen and paper.
The national assessment - which tests reading, writing and language conventions, along with numeracy - transitioned to a fully online format in 2022.
NSW's education standards authority Paul Martin also confirmed students across the state had reported the online portal being slow.
"Some students were able to log on and some weren't. It reached a point where ACARA provided advice to all schools to pause the test," he said.
NSW Education Minister Prue Car asked students and parents not to panic, confirming there was a wide window to undertake the testing.
Victorian Curriculum and Assessment Authority chief executive Andrew Smith reported many students had been experiencing difficulty on Wednesday morning.
Schools that did not experience troubles continued testing as planned, he said.
"We don't have actual numbers yet, but it would seem that there's enough schools for us to be concerned," Mr Smith told ABC Melbourne.
Victorian exams have been plagued by blunders in recent years.
In 2024, a mistake allowed pupils to access questions in advance from instructional cover sheets at the front of online booklets.
The debacle affected 65 of 116 Victorian Certificate of Education exams and led to the sacking of the state curriculum and assessment authority's entire board following a review.