Actually the NZ Prime Minister didn't propose the change because of the Union Flag. He wanted it changed because of how similar it is to our flag. He said he was sick of going to international conferences and seeing a little Australian flag accidentally placed beside his nameplate.
And I wouldn't consider Australia as a whole to be "anti-Brit/monarchy". If we were, we would have voted for a republic when he had a referendum on it almost 20 years ago.