The Kiwi's did fight well in WW II.
The question is: Did they want to?
The logistics of taking over Australia would have made Rommel's supply-line problems in the Western desert seem neglible. According to the Japanese themselves, the goal was maybe to take Darwin. The Kiwis would have been isolated but pretty safe.
That's not to say the Battle of the Coral sea wasn't a glorious victory, or that it didn't turn the tide of the war in the pacific.
As for saving people's asses, the Diggers' timely arrival at Buna after fighting their way over the Kokoda trail saved that fight for MacArthur, who displayed his typical graciousness by issuing a statement saying the battle had been won by U.S. forces with "allies contributing."
Old Diggers still get snaky about MacArthur's passion for flashbulbs.
If you're interested in wartime relations between the Pacific allies, do a web search on "battle of brisbane." You'll be surprised to learn who the australians were fighting.