I think all countries should have that to be honest but New Zealand is Australia's closest ally (literally on the map too :P) and I almost see no difference between Aussies and Kiwis. If NZ let them have the citizenship, Australia should let Kiwis have the same too. Adding this last sentence since I need more words. :P
There is an extreme Anti-Australia sentiment that runs through many Kiwis. They love to move to Australia and whinge about how much better NZ is and how they'd love to move back. They also hate Australia in sport (Because Australia always wins) and supports anyone playing Australia. Kiwis hate Aussies with a passion and shouldn't bother becoming citizens unless they remain loyal to Australia. If they can't do that then bugger off home to your precious NZ.