Suckerpunch:
No, consents can't be issued retrospectively but you could get the owner to obtain a certificate of acceptance.
I've seen plenty of these done, basically a structural engineer inspects and writes a report which from the basis of the cert of acceptance. His inspection can be quite extensive and expensive.
Once it is accepted by the council the insurance company will have no problem at all with it, (assuming all is well) they just treat it the same a CCC.
However in the normal course of events the seller would be advised by the Real-estate Agent to get this before putting the property on the market. I have never seen a buyer pay for one.... But I guess in the market that is Auckland.....