It is not the first circulating New Zealand mule - in 1967 numerous 2c pieces struck with the obverse die for The Bahamas 5c piece were discovered in circulation. The 2004 10c mule was much more subtle however - the design was identical but just 0.6mm smaller - and so went undetected until well after the introduction of smaller change in 2006.
Like the Bahamas 2c mule in 1967, the Royal Mint was also responsible for the 2004 10c mule, producing 6,500,000 2004-dated 10c pieces. The mint sets were produced by the Royal Australian Mint however, and no mules have been reported in mint sets. The exact number of mules produced is unknown but the numerous die markers suggest that it was the product of a single die pairing and their apparent commonness would suggest that the accidental pairing went unnoticed: given this a likely mintage is somewhere in the low to mid hundreds of thousands. While many would have been destroyed when the larger change was phased out in 2006 it is likely that many examples of the larger change were also hoarded, so a reasonable survival rate is to be expected, more so because the mules only circulated for two years.