If you're unhappy with a decision the NDIA has made you can ask them to check it. Once they've checked the decision, if you're still unhappy you can then ask a group called the Administrative Appeals Tribunal to review the decision. Their job is to make sure all decisions are fair.

To start this process off you can fill out the NDIS Application for Review page: https://www.ndis.gov.au/applying-access-ndis/how-apply/receiving-your-access-decision/internal-review-decision.