So, the closest population size is Liverpool-Birkenhead metro population of 2.2 mil, which is serviced by two parks (Anfield - 54K and Goodison Park - 36.5K) which are less than 2 miles away from each other.
Comparing Auckland's size to Liverpool and an Auckland Stadium would have a capacity of 46,000 (close to the 50K I'm suggesting) when just using Anfield. Goodison Park in Auckland would have a capacity of 31,000, which be a decent upgrade to Mt Smart to include Northern and Southern Stands.
The next one on the list, Newcastle-Sunderland, has a metro population of 1.6 mil. Just using St James Park (52.3K) and not the Stadium of Light in Sunderland and that gives an Auckland Stadium a capacity of over 61,000.
Go to the other oval ball code and the Millenium Stadium and Murrayfield Stadium throw a couple of real large spanners into the numbers. Cardiff and the South Wales Valleys metro population is only 1.1m but the Principality Stadium holds over 74,000 - that would equal a Stadium in Auckland of 125,000. Edinburgh metro population is only 780K but Murrayfield Stadium can hold over 67,000 - that would equal a Stadium in Auckland of 160,000. And, you have to remember, those are national stadiums for their "countries" (the continuing debate as to whether they are actually countries is for another day).