Australia's biggest union, the nurses' federation, has condemned the "reprehensible conduct" of the shop assistants' union over sweet-heart wage deals that left hundreds of thousands of retail and fast food workers underpaid.

In a rare breaking of labour movement ranks, the Australian Nursing & Midwifery Federation has attacked its fellow ACTU affiliate in a submission to a Senate inquiry into penalty rates.

Nurses unite. Credit:Angela Wylie

The inquiry was prompted by a 12-month Fairfax Media investigation that revealed how the Shop, Distributive & Allied Employees Association (SDA) struck deals with employers that left more than 250,000 workers paid less than the award, the wages safety net.

The scathing assessment by the nurses' federation federal secretary Lee Thomas and assistant secretary Annie Butler, is the first open attack by current union chiefs since the revelation of the underpayment scandal in 2015/16.