This year 50 per cent of Australian students did their tests online as part of a three-year transition away from the traditional written format.

Connectivity problems on the first day of NAPLAN'S 2019 test schedule left thousands of students unable to complete their written assessment.

Canobolas Rural Technology High School was among the impacted schools, and this week notified parents and guardians that affected students would have the option of resitting the exam on Tuesday.

Central West-based NSW Secondary Principals Council deputy president Craig Petersen on Friday slammed the bungled tests.

Far from just being an inconvenience, Mr Petersen said it was "distressing" for many students.

"We keep saying it's not a high stakes test, but it is. There's pressure from media, parents and schools," he said.

"ACARA [Australian Curriculum, Assessment and Reporting Authority] needs to guarantee it will work as it's intended to work."

Mr Petersen said the adaptive online testing needed a major overhaul before next year's exams.

St Mary's Catholic Primary School co-principal Kerry Maher said NAPLAN online was an outstanding success despite "a couple of technical hiccups that were rectified within minutes".

"We had a really positive experience. We did have extra staff on the ground just in case, but we were thrilled with how it went," she said.