Construction, Forestry, Mining and Energy Union ACT secretary Dean Hall said on Monday he had discovered Australian workers were delivering concrete and other supplies onto the secretive site, contrary to earlier assurances.

''If there is an accident, those Australian workers and their employers have no coverage for compensation of any form,'' he said.

ACT Work Safety Commissioner Mark McCabe said he was powerless to act against allegations of unsafe work practices on the secretive site.

The Canberra Times revealed in November 2011 the federal government had granted diplomatic visas to dozens of Chinese construction workers building the extension to the Yarralumla embassy, placing the crew members out of reach of Australian employment laws.

The CFMEU expressed concerns at the time about whether Australian safety standards could be enforced on the site across the road from the main Chinese embassy complex.

Australia has struck a reciprocal deal for any future work on the Australian embassy in Beijing, the Department of Foreign Affairs and Trade said on Monday night.

A spokeswoman said the department had taken up safety issues with the Chinese embassy at least twice.

Mr Hall said his union had been originally told Australian workers would not be required to enter the site to deliver concrete. ''We were told that wasn't going to happen because they were going to batch their own concrete, but it does happen and we've witnessed it.

''When I was approached by a Chinese diplomat [outside the site], I asked him what happens if there is an accident and an Australian worker is seriously injured or killed. I said there would be big problems around compensation and families wanting someone to be prosecuted.

''His response was, 'If that happened, we'd just claim diplomatic immunity'.''

The deal to allow China to bring in workers on diplomatic visas follows the revelation in the mid-1990s that Australia had bugged the original embassy when it was built in the 1970s.

Mr Hall said China could have employed local contractors as the US did for an extension to its embassy in Canberra.