A man is in a critical condition at a Darwin hospital after being crushed by a 1.5-tonne electrical switchboard at a construction site that a union had previously tried to shut down due to safety concerns.

Key points: The man was injured at the Palmerston police station worksite on Wednesday morning

The man was injured at the Palmerston police station worksite on Wednesday morning The CFMEU raised safety concerns about the site last year

The CFMEU raised safety concerns about the site last year Construction of the station is expected to be finished in mid-2019

The 31-year-old electrician and two others were moving the switchboard into position for installation at the new $30 million Palmerston Police Station on Wednesday morning when he was pinned, NT WorkSafe said.

He is now in a critical condition at the Royal Darwin Hospital.

NT WorkSafe has started an investigation and has issued a non-disturbance notice, preventing access to the switchboard room while it continues.

Earlier this month, the Australian Building and Construction Commission (ABCC) took legal action against the CFMEU, claiming union members had threatened and intimidated officials at the site.

It said that in May 2018, head contractor Sitzler called the ABCC and NT WorkSafe Inspectors to the site after two CFMEU officials repeatedly demanded that the contractor shut the site down, claiming there were safety issues and threatening that the contractor would "pay the price" if it failed to do so.

The new police station has been a key investment project of the Gunner Government and is expected to be completed in mid-2019.

Unions NT voices criticism

Unions NT project officer Adam Giuliani said he had not been to the construction site, but was aware of concerns raised by the CFMEU.

"They have raised concerns about a number of things that weren't getting rectified," he said.

Mr Giuliani also criticised the involvement of the ABCC.

"Statistically you're three times more likely to be killed on a site in the Northern Territory than any other jurisdiction," he said.

"If a company has to use the ABCC because they feel intimidated because they're trying to protect workers, I think they need to have a quick look at themselves."

NT Worksafe investigating incident

Sitzler's director Michael Sitzler said the company was devastated by the incident.

"Our thoughts are focused on the worker and his family at this stage," he said.

The family had been flown up from interstate and workers on the site had also been offered support, Mr Sitzler said.

He said he would cooperate fully with NT Worksafe's investigation into the incident.

"Worksafe attended the scene yesterday after the incident was notified to them … they've done a preliminary investigation and yes we are just fully cooperating with what they require."