It's like you haven't even seen The Castle, where this very constitutional issue is discussed. Acquisition of property must be on just terms with suitable compensation. Similar to the US 5th amendment.
While I agree with you in theory, it's worth remembering the only reason The Castle has a happy ending is because someone who was already wealthy offered to help out people (for free) who weren't.
The first is that a lot of agricultural land is leasehold, not freehold.
The second is that below a certain depth, the Crown owns what's underground and has the right to go through your property to get to its property, whether directly or by leasing it to a mining company.
This happens in a lot of countries and is a problem. That doesn’t mean that rights to private/personal property aren’t guaranteed by law in Australia, just that the law is skirted by monied interests. The farmers you talk about would likely be able to fight this in court if they had means - but such is the nature of late capitalism, money buys you political power and legal clout.
Though genuinely unsure and curious as to whether that's a separate legal (or perhaps media reporting) issue.