Any deficiencies in stopping power can be remedied via the procurement of more modern ammunition types, for example the US M855A1 or SOST ammunition. Whether the Steyr is cleared to fire this ammo is another story. However the issue could be one of optics, as the same sentence that mentioned the 200 metre range also said the weapons were "not powerful enough to identify adversaries" - this could have been a slip up on the journo's part, mixing stopping power and optics power into the one sentence. Do you know what kind of optics the NZDF use on their service rifles?
I haven't seen much detail on the Austeyr's lack of stopping power though, as it has a longer barrel than the M-4 (which seems to frequently be the culprit when one hears talk of the 5.56mm being deficient in killing power) and so is the more capable weapon over long distances. I don't remember hearing much about the USMC's M-16A4 rifles either, and again they have the longer barrel. The M855A1 round is designed from the get go to be fired from carbine-length barrels so it gives better performance in M-4 type rifles, but I don't know if that performance would carry over to longer barrels, I don't know enough about how the burning speed of the gunpowder used etc affects these things.
Someone like Raven could give you a much better answer though, this is all coming from a civilian with a patchy memory.