What about New Zealand?

"New Zealand is about as isolated as you can get in terms of a livable environment, but they never even mentioned it once in the movie."

New Zealand didn't get mentioned in the movie, but it was mentioned in the novel. There were cases of radiation sickness in Christchurch, round the same time Melbourne was starting to report cases.

"And then of course there's the South Pole(Antarctica) as well, while it may not be the most friendly of environments, it would be far enough away from the harmful radiation that would be carried through the wind."

People who fled to Antartica would only last a few months longer than people in Melbourne.

My impression is that, if the author had lived in New Zealand, he would have focused the story there. But he lived in Melbourne.

His rational (I forget where I read this) is that Melbourne is the southernmost "large city" in the world. His definition of "large" included Melbourne but not Christchurch, which is/was much smaller. Any atlas has the figures.

So the novel points out that there are indeed other places south of there in which people could live longer. But they were smaller and didn't count as much. More importantly, people would think about traveling there to get a few more months of life, but never do it because it would be too much work to get too few weeks.