The court has found Aldi breached employment laws by directing employees to begin work 15 minutes before their rostered starting time.
Workers at a Sydney distribution centre were required to undertake a number of tasks before clocking on, including completing safety checks on forklifts, checking communication devices and undertaking a group warm-up activity.
Aldi claimed it was not underpaying its workers and that employees were only expected to be ready to commence work at the start of their rostered shifts.
But Judge Douglas Humphreys said he was satisfied there was a "clear implied direction" that employees had to arrive early and undertake those tasks, and that a consistent failure to comply would lead to disciplinary action.
"There was no personal benefit to the employee in the activities carried out. Each was to the benefit of the employer," he said in his findings.
"In these circumstances, the court is satisfied that the activities carried out constitute work."
The SDA retail and warehouse union claims Aldi owes its distribution centre workers up to $10 million in unpaid wages for working an extra 10 minutes per shift.
It says the decision could affect about 4000 current and former workers, a claim disputed by Aldi.
The company said it respected the court's decision and was determining the payment owed to four employees directly impacted by the proceedings.
"We are reviewing the implication of the court's decision on other employees across our business and will seek to apply the principles of the court's decision fairly to any other affected employees," an Aldi spokesperson said on Wednesday.
"Numbers quoted by the SDA are significantly inflated and are not representative of the number of employees we believe may be impacted by the decision."
The SDA said it was still working through calculating individual back-pay amounts for its members.
"Aldi joins a long list of large employers found to have underpaid their workers," NSW branch secretary Bernie Smith said.
"Unlike the way the multinational likes to present itself in its advertisements, it turns out Aldi is not good and not different.
"Multinational companies operating in Australia can't set their own rules."
Judge Humphreys is yet to make any orders regarding penalties. The parties are scheduled to return to court on Friday for a directions hearing.
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