This is the name of a classic arcade game in which the task is to hit a mole over the head as soon as it pops up on the screen. The game has been varied in many ways. For example, instead of moles you might have dinosaurs. It has even been made educational so that instead of moles you have letters of the alphabet and such like.
Whack-a-mole (often unyphenated as whack a mole) has acquired a figurative use where the moles become assailants or even problems that you hit over the head as soon as they appear. The idea is, however, that this repetitive action is ultimately futile because you may down individual moles but they keep coming at you.
It is an Americanism which is perhaps gaining ground in Australian English. We are used to the arcade game so there is no reason why it wouldn’t gain traction. It turns up in the expression play whack-a-mole meaning to engage in pointless opposition.
It is perfectly possible that this expression will take off but at the moment it has low frequency in Australian English.