Yes. That's correct. For Ph.D. students in Australia, the funding is provided by the federal government. The Ph.D. scholarships are indexed every year. You can look up the historical Ph.D. student stipends on the department of education website. They go back many, many decades. You can see that they've been going up regularly for the whole time.
As for post-doctoral fellows, the salary is set by the institution that employs them. As a post-doctoral fellow in Australia, you're covered by the enterprise agreement just the same as any other employee of the university. Your pay is indexed according to that enterprise agreement. You will not have any issue with your pay decreasing over time. Hopefully, if the pay rate that's been agreed to with the employees goes up, then you will beat inflation.