Manukau is not a city. The centre contains a Westfield, public offices, substantial police and justice facilities (supply and demand), two pubs, two cafes, a Warehouse, a smaller shopping mall and various factories, call centres and lunch bars sprawling into the distance. Plus a theme park.
That is a suburb which happens to have had local government concentrated there.
Ditto Takapuna "North Shore City", Henderson "Waitakere City", etc.
Actually Manukau is a city. Covers over 55,000 hectares and with a population of 335,000 is the 3rd largest city in NZ. This "suburb" as you refer to it, is the Manukau CBD.
Ditto North Shore, and Waitakere etc