Private contractors building the Sydney Metro Northwest have been accused of trying to slash the pay and conditions of skilled tradespeople using a controversial pay agreement like the one Carlton United Breweries used to slash worker salaries.

A union investigation has found three electricians employed on the $8.3 billion NSW government-funded project were offered a minimum base wage with UGL plus a $3360 monthly "bonus" from a second business. The workers said they were then asked to vote on a workplace agreement that would cover hundreds of electricians in three states.

The Electrical Trades Union said the company had tried to use the same legal loophole that led to the Carlton and United Breweries dispute last year, in which a handful of workers were forced to rubber-stamp a workplace agreement used to slash the wages of hundreds of other employees.

Electrical Trades Union secretary Dave McKinley said UGL through a wholly-owned subsidiary had used a similar tactic to try to pay lower wages to hundreds of electricians working on the construction of the Sydney Metro Northwest.