I was recently interviewed for a postdoc position. The next day I got an email from the Uni (in Australia) to do a background check. Having waited for the check for two weeks, I was wondering if the background check at this stage means I am the selected one, or I am still in a shortened pool.

Thanks.

I don't think you can interpret it either way. They equally could have background checked the entire list or just you. You honestly can't tell but I wish you all the best.
Fingers crossed you get it!