A spate of brain tumours among staff has forced Melbourne's RMIT University to close part of its business school and test for radiation emissions from rooftop phone towers.

As staff reacted with shock, the university yesterday shut the top two floors of the Bourke Street building and ordered more than 100 employees to work from home for the next fortnight.

The closure follows the discovery of five brain tumours in the past month and two others in 1999 and 2001. Two were malignant and five were benign.

WorkCover has launched an investigation and RMIT has promised its own inquiry.

The academics' union last night expressed concern that the tumours were caused by the communications towers on the roof of the former Tivoli Theatre site.

National Tertiary Education Union state secretary Matthew McGowan warned that anecdotal reports from hastily arranged staff meetings yesterday suggested the number of people affected would grow.

"You have to ask some pretty serious questions and we're obviously concerned that it could be linked to the tower," he said.

"This would appear to be much more than coincidence and RMIT has a responsibility to leave no stone unturned in seeking the truth."

Five of the seven affected work on the top floor of the 17-storey building. All except one have worked in the building for at least a decade.

An RMIT academic who did not want to be named said staff - the 16th and 17th floors are home to offices of senior management and lecturers - were "in disbelief, concerned and upset" as they attended meetings and left the building late yesterday.

Medical experts said no definitive link had been proved between mobile phone tower radiation and cancer.

Australian Medical Association president Mukesh Haikerwal said there was no proof of a connection but "if you get clusters of disease it's sensible to investigate."

Dr John Gall, from private health company Southern Medical Services, which has been called in to assess the sick, said last night three of those affected had tumours showing symptoms consistent with radiation.

But he said there was no causal link with the building based on preliminary observations.

A spokesman for state Health Minister Bronwyn Pike said WorkCover would investigate the matter and the Department of Human Services would provide any expertise needed.

RMIT chief operating officer Steve Somogyi said testing was carried out on the building after the first two of the seven tumours were reported in 1999 and 2001. It found radiation and air quality levels within recommended guidelines.

"We value the health and safety of our staff and students very highly. The incidence of illness is disturbing and we shall continue to check for any possible cause connected to the building," Mr Somogyi said.

But RMIT union branch president Jeanette Pierce said the university agreed to shut the two floors only after being pressured by the union. "I'm a bit mystified that the university wasn't planning to vacate and that we had to make the point that they needed to vacate those two floors," she said.

There are more than 160 mobile phone towers in central Melbourne alone. A Telstra spokeswoman said last night the company had two towers at the Tivoli site, but both met health and safety standards and were tested regularly.

"An enormous amount of medical research has been conducted without any substantiated evidence of a link between mobile phone technology and adverse health effects, including cancer," she said.

RMIT management emailed all staff and students late yesterday and said health check-ups and counselling would be made available. About 600 staff work in the building.

Mr McGowan said shutting the two floors should be just the first step. "We think they should be testing all staff who have worked on those levels and not just for tumours. We need to understand what are the health risks that people are suffering," he said.

A help line for students and family members is available on 1800 155 945.

Tanya Stoianoff, the executive director of the Mobile Carriers Forum, which represents mobile phone companies, said there was no credible scientific evidence of health effects from living or working near a mobile phone base station.