The Wellington Regional stadium needed to be a 2 tier rectangular 30k stadium, where you could open the bottom tier only. Far too late now. Time machine best option :/ Its pointless building stadia for a once a year all black test presumably.
I can't really think of a location that you could build a 10-12k stadium at with good public transport options for porirua and the hutt + wellington.
https://www.skystadium.co.nz/fileadmin/Documents/Annual_Reports/WRST_AR25.pdf
Any private option will really undermine the viability of Sky Stadium. Still in surplus - but marginal.