I watched a reality show on the Australian border controle where they destroyed an object in a mail package and found drugs. The destruction wasn't really subtle, you couldn't use the object afterwards (it was a bowl). Them doing it was legit, since the object indeed contained contraband. However, if the sending would have been legit, how liable would they have been for destroying your property? The same applies when customs go to your luggage and destroy something, can you get reimbursed?
Most customs agencies in modern countries have some method for you to claim in the event of damage caused to your posessions / luggage / souvenirs.
For example, since you mention Australia, on their Australian Customs and Border Protection Service website they have a publically published Policy for dealing with allegations of damage at Cargo and Container Examination Facilities. This includes information on how to make a claim, what evidence to provide, and the reasons that they may have for deliberately causing damage, and when this would make them not liable for damage/destruction of your property.
So yes, sometimes they can be liable, and there is usually a means of claiming for this.
I agree with the initial response - that there are probably laws (variations on the same - for each country)but hard to enforce or redress. I recently experienced the same situation - unnecessary damage to property by customs.
I was shipping a busker Organ (street organ)from Panama to the UK for repairs. The recipient discovered the packaging was in good shape ans therefore assumed a safe delivery. But once opened saw that parts were severely damaged: Pictures taken showed - The top which was screwed in place had been ripped off. The top was pried off when screws could have been unscrewed and lifted off. A protective grill for shipping was removed and the decorative protective grill broken and organ pipes were crudely removed There were also other minor scratches and dents to the fine wooden finish. The repairs for which I sent the organ were quickly done but the damage caused by Customs took much longer, required fine craftsmanship and cost much more.
All this and actually no recourse. Panama denied opening the package and blames the UK. and the same in reverse in the UK, blaming Panama. One even accused, that since I was shipping the organ for repairs that perhaps these were the damages that needed repairing.
The US Customs and Border Protection's website describes the process for filing a claim for "property damage or loss, or personal injury or death".
If you would like to make a claim for property damage or loss, or personal injury, or death resulting from the negligent acts or omissions of an employee of U.S. Customs and Border Protection (CBP), you may file an administrative tort claim against the agency. CBP processes administrative tort claims in accordance with the Federal Tort Claims Act (FTCA), 28 U.S.C. § § 2671, 2680.
Claimants who wish to file an administrative tort claim against CBP may complete a Standard Form 95, Claim for Damage, Injury or Death (SF-95) (see attached) and submit it, along with supporting documentation, to the CBP port of entry or Border Patrol station nearest the geographic location where the incident is alleged to have occurred.
It seems like a fairly straightforward process, though you'd likely need to consult a lawyer first for specific advice, which raises the entry-barrier somewhat. Of course this is a good reason to have insurance for high-value goods and not to import sentimental items out-of-sight (at least that way you can personally instruct a customs official on how to safely open or disassemble an item).