About time there was another domestic airport north of the harbour bridge. I think Whenuapai would work well. The Civil/Military model works well at Woodbourne, I cannot see why it cannot work at Whenuapai.
An airport in that area would help with Auckland's congestion. People flying to/from Auckland will have the choice at several locations of flying to/from either the south of Auckland or the north of Auckland depending on where they need to go or where they live.
I think it's a great idea.