The boundaries of all the states except South-wales and Victoria border are same divided with a straight line. The reason behind the border not being straight is that the majority of the border between Victoria and New South Wales is formed by the River Murray (Interestingly the river is actually in New South Wales entirely, instead of the border being right through the centre of the river). It doesn’t form the border between South Australia and Vic/NSW or any of the other states as it either doesn’t run through the states or, like in South Australia, cuts right through the initial territory designated for the free colony. As they didn’t want to form another state they just kept SA as it was and had the river running through it.
But essentially, the border is formed by the Murray, which, as a river, naturally isn’t a straight line.
