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.