New Zealand's flag became official on March 24, 1902, but the design was already three decades old by then. It had been picked for naval use in 1869 and simply stayed.
Earlier flags
The first flag recognized for New Zealand was that of the United Tribes of New Zealand, adopted in 1834 by Māori chiefs as European settlement and shipping increased. It carried a red St George's Cross, a blue canton with a smaller white cross, and four stars for the Southern Cross. British ensigns followed the signing of the Treaty of Waitangi in 1840.
The design has been argued over on and off ever since. The 2015-2016 referendum put the question directly, offering a replacement without colonial symbols, and voters kept what they had. Opinion remains split between people who want a flag free of colonial imagery and people who value the continuity of the current one.
The Union Jack and the Southern Cross
A blue field, the Union Jack in the canton, four red stars edged in white to the fly. The blue is the Pacific. The Southern Cross fixes the country in the southern hemisphere, which is why the same constellation turns up on several flags in the region. The Union Jack records the constitutional link with Britain.
Ensigns and usage
The flag flies at government buildings, national events, and military installations, and protocol prohibits its use in disreputable contexts. Waitangi Day and ANZAC Day are the days it is most visible. Merchant vessels fly the New Zealand Red Ensign, the Royal New Zealand Navy a white ensign, and the Air Force a blue ensign with a red kiwi in flight.
The Australian resemblance
New Zealand's flag and Australia's are easy to confuse. Both use the Union Jack and the Southern Cross, and both descend from British Admiralty guidance that shaped ensign design across the Commonwealth.