The disruption occurred on Wednesday morning as students were scheduled to sit the writing component of the annual assessment online.
An Australian Curriculum, Assessment and Reporting Authority spokesperson said the issue 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," the authority said.
"Schools have been advised to pause testing while this is being investigated."
The outages have been reported nationwide, although the full extent is not yet clear.
More than 1.3 million students in Grade 3, 5, 7 and 9 were set to begin testing on Wednesday.
Victorian Curriculum and Assessment Authority chief executive Andrew Smith said many schools had difficulty loading content in the platform.
Schools that have not yet experienced difficulties can continue testing as planned, he added.
"ACARA have asked that schools pause testing while the issue is investigated, and they will be given advice as through the course of the day as to when they can resume testing," he told ABC Melbourne.
"We don't have actual numbers yet, but it would seem that there's enough schools for us to be concerned.
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.
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