Attendances plus ratings gives you your total audience. NRL is the clear winner. AFL has bigger crowds because they have first class suitable stadiums for all their teams. Many NRL teams' attendances are capped by ground capacity.
Here's some ways the NRL could boost the league's average attendance up closer to 30k...
New Olympic sponsored 65k Suncorp, Broncos can now push up towards a 50k average and beyond
Dolphins continue to grow their fan base to 25k and beyond.
New Auckland stadium getting the Wahs past 30k
Roosters continue to build towards 25k and Souths move back to Allianz to bring their average up over 20k
Dogs and Tigers get that 45k Liverpool stadium built which would bring them both up over 20k and building over 25k in good times.
Sharks ground brought up to 18k, then they, Dragons, Manly play key marquee fixtures at Allianz to bring the average for all three up over 20k.
Canberra, new stadium bringing the Raiders' average to 20k
Storm and Western play a marquee fixture at Docklands and Optus respectively to push averages for both towards 25k.