Can I change my specialization from pharmacy to IT while pursuing my Masters in Australia? Will I face any visa or PR issue?

You can come in any department but the point is you can't handle IT after pharmacy, logics and all, everything is different and typical logics but why do you want to change your field from pharmacy? There are lots of ways from pharmacy just do pharmacovigilance, if you want more details regarding th

You must meet academic requirements before you could transfer to another program. Both the programs you mention are entirely different, so chances are you won't be accepted by any university.