Australia is a small continent isolated from a big world. Australian animals did not have as much competition as rabbit had during his evolution. So rabbit from Eurasia in Australia is like Earth bacteria wreaking havoc in some small oasis of life at some distant planet. It works that way, but it won't work other way. It took hundreds of thousands of years of constant evolution for COVID virus to break through our immune systems. Martian bacteria won't have that time.

Now if some otherworldy life had billions of years of evolution similar to ours or exceeding it, well, it might present some threat.