It's really tough to predict isn't it? 6am is a reasonable time and it's a huge game, but since it shares the weekend with the AFL and NRL grand finals it'll struggle to get much coverage. You'll probably end up with a situation where a million people watch it in New Zealand and 400,000 odd in Australia.
6am on a Sunday morning (that feels like 5am due to daylight saving changeover) is still a time for mostly die hard fans. It's obviously a more conducive time to attract an audience than the middle of the night but still not great.
Essentially it will be people who change their routine to watch the game as opposed to it being at a time when lots of people will be watching TV and it's attractive for sports fans.
I think 400k or so sounds about accurate.