Mr Corbell revealed the planned investigation - in which witnesses' evidence will be protected from defamation and other legal action - in the wake of the fourth workplace death in Canberra since December. Three have been in the civil and construction sector, and the fourth involved a painter.

''That is a tragic and unacceptable toll and, in the government's view, we need to look more closely at why we are not seeing sufficient compliance from the civil and construction sector when it comes to workplace safety; why it is that we are seeing this level of death and injury,'' he said.

Concreter Ben Catanzariti, 21, was working at a Kingston Foreshore construction site on Saturday morning when he was struck by a 39 metre boom and died at the scene.

ACT Work Safety Commissioner Mark McCabe said yesterday the site remained shut, and he did not know when it would be reopened.

''We still believe at this stage that it's an issue about equipment failure, but it's very early stages, so it's still hard for us to say definitively what the issue was,'' he said.

''The next issue will be when we open up the site for them to go back to work and when we clear those concrete trucks to go back into operation.''

Mr Corbell said authorities could not yet rule out human error in relation to the death, but emphasised he was not making any comment about the firms involved in the weekend accident.

''But in general we still see a tendency in some parts of the industry for profit margins to be traded off against workers' safety and that is an unacceptable situation and one that we will get to the bottom of through this investigation,'' he said.

Master Builders ACT executive director John Miller said it was a tragic accident, but he thought the Work Safety Commissioner should be allowed to finish his investigation before anyone decided whether a full inquiry into the industry was warranted. ''Obviously it is a very unfortunate set of circumstances where we have had a number of incidents over the [past] eight months, but I think it is fair to say … there has been no common thread there.''

Opposition industrial relations spokeswoman Vicki Dunne said the weekend tragedy had to be thoroughly investigated, and the Liberals were open to discussing the terms of reference of a broader inquiry.

Mr Corbell said the inquiry would likely be held under the Work Health and Safety Act and a panel of investigators would have the power to call for documents, receive evidence and question people. Witnesses' evidence would be protected from defamation or other legal action.

The Justice and Community Safety directorate was drafting terms of reference, which would then go to the Work Health and Safety Council - which includes industry and union representatives - for consultation.

''I have separately asked the Work Health and Safety Commissioner to advise on what steps he believes he can take to strengthen enforcement action in the sector,'' he said.

Mr McCabe is expected to be one of a number of people conducting the inquiry, along with an outside person.

Mr Corbell expected the inquiry would call for submissions and meet employer and union representatives and occupational health and safety practitioners, although he did not think there would be any public hearings. The report and its recommendations would be made public.

He said there had been some high profile near misses as well as the recent deaths. ''We have had the collapse of the DEEWR pour a couple of years ago when the DEEWR building was being built in Civic, we have had the collapse of the bridge on the Barton Highway with the Gunghalin Drive Extension,'' he said.

He expected to be able to formally announce the investigation, which would take three to four months to complete, within a fortnight.