In the midst of the current flag debate, the alternative looks like a 'Rugby Flag' and is not a suitable flag for a nation-state.
Also, whether you like it or not, New Zealand is a British Colony and we should be proud of that fact? The Union Jack is a symbol of our identity. We are British, and our values are based on British values.
Third, the argument that New Zealand's flag is similar to Australia's is a weak argument. There are many nations with similar flags: the Netherlands and Luxembourg; Cuba and Puerto Rico; Romania and Mali. Just because other people don't know or notice the difference is not our fault; that's their problem; and we shouldn't have to change our flag just to make it easier for other people. Both New Zealanders and Australians know the differences in their flags.
The flag referendum began just after we won the world cup, and this affected people's better judgement as they believed that ruby was a main part of who we are as New Zealanders, thus the majority of the flags were ruby related.
Also all the flag choices were terrible, i believe that instead of letting the public make a flag the government should have asked New Zealand's leading artists to create an outstanding and memorable flag. But instead they came up with 4 ugly flags that look like a logo for a two dollar shop, not the kind of look we want as a country.