In Australia, to register a .com.au domain name, you have to demonstrate some affiliation eg a registered company of that name, or a product that has been trademarked with or is trading under that name.
On the other hand, if you don’t want to spend that money up front, you can wait, knowing that nobody else can register the domain (unless they spend a similar amount first).
But when you register the domain you agree to the policy I've pasted below.
So you can start using the domain straight away, but if, before you have registered a matching business/company name or trademark, somebody else can demonstrate eligibility to use that domain, they can challenge your registration and you'd have to hand it over.
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Policy Acceptance
By submitting this application for a domain name, you confirm that you are eligible to hold the domain name set out in this application, and that all information provided in this application is true, complete and correct, and is not misleading in any way. If any of the information is later found not to be true, or is incomplete, incorrect, or misleading in any way, or if you have submitted this application in bad faith, the domain name license can be canceled and you can permanently lose the use of the domain name. I certify that I understand the agreement and that domain names have a close & substantial connection to the person/business that it is intended to represent.
Not surprising, considering there are at least 4 auDA accredited registrars who specialise in domain parking[1], which is (allegedly) against the rules.
[1] https://www.auda.org.au/accredited-registrars - see drop.com.au, terrific.com.au, yexa.com.au, fabulous.com.au
Here are a few examples, all definitely parked domains - but auDA allows it:
television.com.au
bank.com.au
remotecontrols.com.au
modem.com.au
tweet.com.au
homephone.com.au
homepage.com.au (This domain is for sale, $80k)
It would be great if auDA enforced their policy...