The rules around sick leave for more than a million shift workers have been upended in a controversial industrial umpire's decision that has employers worried they may face a massive bill for backpay and sparked an urgent legal intervention from the federal government.

The Fair Work Commission recently pulled the rug on the standard practise employers used of providing an average of 76 hours of paid leave based on the average 7.6-hour work day and the national minimum standard of 10 days of sick leave a year.

Peter Kelly is one of the workers affected by the landmark sick leave ruling. Credit:Peter Braig

The Commission's ruling, which centred on a pay deal for employees at pharmaceutical company AstraZeneca, found a worker's typical shift should be used to calculate the total number of hours meaning a shift-worker who does usually a 12 hour shift should receive 120 hours of paid leave per year.

The ruling, in favour of the Australian Workers Union, has alarmed business groups and employers who have now turned their attention to a fresh test case to be brought in the Federal Court. That case will use conditions at manufacturer Mondelez in Tasmania to argue the commission's rationale should be overturned.