The Australian Services Union and others are fighting to stop NDIS providers from classifying workers as lower-paid home care staff, while claiming the full price from the scheme and pocketing the difference.
The union estimates one in ten providers are ripping off disability support workers by misclassifying them at a lower award rate, with some losing $9 an hour on a regular shift and $20 per hour on public holidays.
The Fair Work Commission is expected to hear the final statements on Thursday or Friday.
Natalie, a worker from the Hunter region, estimates she was underpaid by her previous employer by more than $18,000.
The 21-year-old said the experience left her questioning her future in an industry she found fulfilling.
"It's disgusting, they've (previous provider) ripped off the clients and they're denying any wrongdoing," she told AAP.
"Definitely at that time, I felt like it kind of ruined disability work for me."
Natalie, who requested her last name not be used, said she was now working happily for another company that valued their staff as employees and people.
The union's NSW and ACT secretary Angus McFarland said the commission can close the loophole allowing the underpayment by amending the social, home care and disability services award.
This would ensure all workers providing NDIS-funded services are properly classified and not paid at a lower rate.
"Dodgy providers are the grinches stealing Christmas from disability support workers by underpaying them - and this outrageous practice extends to regular work days too," he said.
Mr McFarland said it was unsustainable for unions to continue playing "whack-a-mole" on NDIS providers not doing the right thing.
"The disability support sector faces one of the highest attrition rates in the country, and wage theft significantly undermines efforts to attract and retain skilled workers," he said.
The commission is expected to hand down a ruling in early 2025.
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