The top of D Block at Whanganui UCOL was demolished yesterday.
It had contained a lecture theatre and life drawing room, the head of the polytech's Safer UCOL Buildings Project, Katrina Langdon, said.
It was the most unsafe of the buildings vacated by arts students last year, and also had the least historic merit, she said.
Work started last week and will be finished in seven weeks.
No replacement building and no further demolition was planned, Ms Langdon said. She was unsure whether any more of the vacated fine arts buildings would come down.
"Ideally, we would like to see them kept, but safety will always be our pre-eminent concern."