FIFA stadium requirements for World Cups are Group games 40k minimum, knock out games 60k minimum
That means currently Eden Park is the only ground in NZ that would qualify.
With additional temp seating, Westpac would go pretty damn close. Permanent capacity 34,500
Crowd for Peru game in 2017 was 37,000 and not quite a sell out from memory.
Worse case 2 pools, and 6 pool games at Eden Park over 2 weeks.