Easy one to mix up:

• The NDIS (National Disability Insurance Scheme) is the scheme itself: the funding and supports that participants receive.
• The NDIA (National Disability Insurance Agency) is the government agency that runs the scheme: it makes access decisions, approves plans, and manages funding.

So you're a participant in the NDIS and it's the NDIA that approves your plan. When people say "the NDIS decided...", they usually mean the NDIA.