The Federal Court has backed a negotiating tactic which allows construction companies to sidestep unions on agreements for new projects.

Building firms will now be able to forge agreements with the first few workers on a project, and then apply those conditions to all future employees.

A recent Federal Court decision in Perth has backed the tactic, which will mean big changes at the start of new projects across the country.

The Construction, Forestry, Mining and Energy Union (CFMEU) is reportedly furious at the laws, which disconnect it from its members.

Australia Petroleum Production & Exploration Association (APPEA) advisor and former Labor minister Martin Ferguson says the move to reduce the power base of his old party will help investment for the groups he now represents.

The ruling supporting the practice comes from a case involving big builder Leighton Holdings’ John Holland division.

The CFMEU opposed agreements negotiated with just three workers over massive Perth children’s hospital upgrades, and took the case to court.

A Fair Work Commission representative approved the agreements, but the full bench of the commission sided with the union.

The bench’s decision has now been overturned Judge Antony Nicholas Siopsis, who said the full bench “fell into jurisdictional error”, as there were no laws governing how many workers should be able to negotiate the first deals.

He acknowledged the issues with such limited negotiations, but said that they would have to be addressed with legislative change.