The Fair Work Ombudsman (FWO) has started legal action against the construction union and one of its officials for allegedly blocking workers and causing safety risks on a Perth work site when he climbed a 50-metre crane and refused to come down.

The official also allegedly obstructed a generator at the site the following day.

The Construction, Forestry and Maritime Employees Union (CFMEU) and official Stephen Parker could have to pay a total of more than $450,000 in fines if found guilty.

The FWO started investigating after a complaint by head contractor Multiplex at the 'Nine The Esplanade' project in the Perth CBD in November last year.

The ombudsman alleges Mr Parker used his union right-of-entry permit to access the site on November 27 and told the manager he had safety concerns about the crane.

He then allegedly climbed up the 50m ladder to the crane cabin, despite the manager urging him not to. 

Mr Parker then allegedly stayed in the cabin for an hour. The crane did not operate during that time.

Safety risk

The next day, the ombudsman alleges Mr Parker came back to the site and this time asked for a demonstration of a "pre-start check" of the crane's generator. 

To do that the generator had to be turned off.

Mr Parker then allegedly stood in front of the generator after the check had been conducted and stopped the site manager from restarting it, saying it should not be turned back on until "issues" were resolved. It's unclear what those issues were.

Mr Parker allegedly only moved out of the way when a technician arrived to perform a routine service and maintenance of the crane, and the generator was turned back on.

The ombudsman says the generator was off for two hours, meaning the crane was out of action and creating a safety risk for workers at the site who relied on the crane to lift a rescue cage if there was an emergency.

The CFMEU and Mr Parker are charged with breaching sections of the Fair Work Act, which prohibits acting in an improper manner and/or intentionally hindering or obstructing persons at a work site, and which requires right-of-entry permit holders to comply with occupational health and safety requirements on work sites.

'History of non-compliance'

Acting Fair Work Ombudsman Michael Campbell said all workplace organisations, including unions, have legal obligations to behave in a proper manner and meet health and safety requirements.

"We expect union officials to follow the law," Mr Campbell said.

The matter is due in the Federal Court on November 29.

The CFMEU has been contacted for comment.