Former prime minister John Howard has been heckled by union members protesting against the Fair Work Commission's (FWC) decision to cut Sunday penalty rates.

Mr Howard's office said he was walking in the Sydney CBD when he ran into the protesters, with some surrounding him while shouting.

Union members "booed" the 77-year-old with some calling him "a grub" and "a scab".

"We're workers, united, we'll never be defeated," the protesters yelled.

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Police noticed the incident and sought to move the protesters while accompanying Mr Howard and one staff member.

A spokesman from his office said Mr Howard was fine and not bothered by the heckling.

Prime Minister Malcolm Turnbull condemned the union's "thuggish harassment" of Mr Howard.

He also called on Opposition Leader Bill Shorten to condemn the union activity.

The FWC ruled Sunday and public holiday penalty rates would be reduced for full-time and part-time workers in the hospitality, retail and fast-food industries.

The Australian Council of Trade Unions (ACTU) said nearly 500,000 people, including some of the country's lowest-paid workers, would lose up to $6,000 a year.

Labor is preparing to campaign hard on "untenable" penalty rates cut.