jonathan18:
I think a number of these situations of two Countdowns so close to one another at least originated with Woolworths’ purchase of Countdown (where there had been one of each company); that they have kept both stores open is no doubt to exclude the competition. Another example of this is Johnsonville, where one is in the mall and another just opposite.
The larger of the two in Johnsoville (the one not in the mall) was originally a "Big Fresh" store - the ones with the singing/dancing animatronic chickens and vegetables. Off-topic I know, but interesting article about Big Fresh history here: https://thespinoff.co.nz/society/07-04-2017/remembering-big-fresh-new-zealands-greatest-supermarket-of-all-time/
(edited to clarify)