New Zealand flag(s)!

I finally had a chance to do a little research on the NZ flag...

I knew the official flag had a blue background, but as we took the ferry past the naval shipyard the other day, I noticed one of the vessels was flying a WHITE background flag.  Then, we rode on the back of the ferry on our return, and I saw our ferry was flying the flag with a RED background!  Mystery...

Answers from the NZ Ministry of Culture and Heritage:

• New Zealand Government ships must fly the New Zealand Flag. New Zealand ships other than Government ships may fly the New Zealand Flag, the New Zealand Red Ensign or any flag authorised by either the Sovereign or the Governor-General.However, it may be flown on land in places or on occasions of Māori significance. The Flags, Emblems, and Names Protection Act 1981 allows for the continuation of this traditional practice.
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• Ships and commissioned shore establishments of the Royal New Zealand Navy fly the New Zealand White Ensign. This flag has the Union Jack in the first quarter, on a white background. The Southern Cross, represented by four five-pointed red stars, appears in the fly.