It depends on the Uni.
Firstly, most unis (or UAC/VTAC etc) have a mechanism to convert uni marks/GPA etc into an equivalent ATAR.
Eg imagine you have already been at two different unis - one for six months, the second for a year - you will now have 3 possible values, your high school HSC ATAR, your ATAR equivalent from tertiary attempt 1, your ATAR equivalent from tertiary attempt 2.
Some unis will not count your tertiary attempt 1 because it is less than 1 year.
Some unis will use the best of all three.
Some unis will average the high school with tertiary attempt 2.
Moral of the story – ask the specific uni