It looks like the site for the Wellington supercharger has been decided and shouldn't be far off!
http://investoreproperty.co.nz/documents/Investore_Property_Limited_Interim_Report_HY20_131119.pdf
The TL;DR summary is that they're being installed at Countdown Johnsonville
http://investoreproperty.co.nz/documents/Investore_Property_Limited_Interim_Report_HY20_131119.pdf
The TL;DR summary is that they're being installed at Countdown Johnsonville