Attorney-General Christian Porter has left workers sceptical of his promise to lift wages and tackle exploitation after praising a union that stripped workers of more than $1 billion worth of wages as the "gold standard" of "cooperative" workplace relations.

Speaking to business leaders in Canberra on Thursday, Mr Porter named the Shop, Distributive and Allied Employees' Association (SDA) - which signed up workers to enterprise agreements that left them on as little as $8 an hour - as a model union.

Christian Porter has described a union that stripped workers of $1 billion in wages as 'the gold standard'. Credit:AAP

"You speak with the major employers, they have a very good relationship with the SDA. It is constructive, it's sensible," the Morrison government's industrial relations minister said.

An award-winning investigation by The Age revealed in 2016 that a quarter of a million retail and fast-food workers were underpaid more than $300 million a year over five years by employers including Woolworths, Hungry Jack's and KFC, on enterprise agreements negotiated by the SDA.