But in Australia,(as in "aus.gardens") almost every plumbing fitting used
for showers for the last couple of hundred years has been 1/2" BSP.

If you have the other...

Give it a bit of a clean with a wire brush and a bit of elbow grease if
there is a lot of shit, have a look at the thread and see if it's damaged,
see if the end of the threaded pipe has been cut square or you are trying to
cross thread it.

Hope this helps,
Peter