The International Date Line nominally runs down the 180 degree line of longitude, vertically from pole to pole.
In places like Tonga and other Pacific island groups the line deviates to ensure that all islands in the group share the same time zone.
In Tonga, they make a big deal of the Date Line when they say that one side is ,say, Monday and if you step over to the other side it's Tuesday!
But thats just a tourist attraction, really, all tonga is on either Monday or Tuesday.
By the way time zone lines never run "horizontally " in Australia or anywhere else, ever!