I think a lot of confusion arises because of people conflating statute law and common law. I am not (yet) a lawyer, but in some ways, that can be an advantage because I think I can still explain things in 12yo language.
If anything in this answer is wrong, I trust one of the several lawyers on the forum will correct any errors. Please do!
Statute law is what most people think of as "the law"; it consists of Acts and Regulations, and is the law made by Parliament. You can look up the relevant Act and it explicitly says what you can and can't do.
But there is also an enormous body of "common law", which - whilst it may not be written explicitly as "Thou shalt..." - is still binding law, just as much as statute law. Common law is huge, and governs much of contracts, torts (negligence, defamation, etc.), and many other areas of law. It consists of all the legal principles established by decisions of Courts over time, binding precedents, interpretations, and so on.
In this instance, nowhere in Australia (that I'm aware of) is there a statute that says "Sellers have to disclose if there have been murders on the property", that's true.
But... in both NSW and Victoria, there have been cases where purchasers have sued with regard to non-disclosure of a suspicious death, and in both cases, the purchasers won. Both won based on the common law principle from contracts that, if they had known, they wouldn't have entered the contract, and therefore, they had a reasonable expectation of disclosure. (Because there is a common law principle that if you know things that would reasonably cause somebody to not want to enter into a contract with you, you have to disclose it.)
Therefore, the common law in NSW and Victoria - as a result of these decisions - is that deaths that may cause a property to be "psychologically stigmatised" have to be disclosed. See:
http://www.rklawyers.com/areas-of-practice/25-property-law/57-vendors-must-disclose-all-material-facts-about-property.html
If either of those cases had gone as high as the Court of Appeals (or equivalent) in those states, or the High Court, it would arguably be common law nationwide, but neither of those cases did go that high, so presently it is only common law in NSW and Victoria.
Having said that, there are now two precedential cases that would be "persuasive" (but not binding) on interstate courts in the absence of more persuasive authority, so it is likely that the result would be the same if similar cases arose in other states.
Statute law can override common law, but generally not retrospectively. So the legislatures of NSW and Victoria - or any other state - could make an explicit law saying "There is no requirement to disclose murders or suicides when selling a property", and that would then be the law and override common law (but would not change the outcomes of already-decided cases).
The position could also be reversed if a case goes to a higher court - the Court of Appeals or the High Court - and is decided differently, i.e. if one of those courts found that there's no obligation to disclose past murders or suicides.
In summary: it is certain that you currently have a common law obligation to disclose past murders and suicides in Victoria and NSW; it is likely that you would be found to have a common law obligation to do so in other states. (Unless they already have statutes saying that you don't.)