Your wife's Visitor Visa expiry date should have been extended to 25 September - did you receive an email from INZ yet? The INZ website says this about the emails it was sending last week:
"The email will be sent to the most recent email address that the visa holder provided Immigration New Zealand".
Did you check that email address? If you haven't received an email, it might be worthwhile calling the Contact Centre just to make sure of her visa extension - I know they are taking a long time to answer the phones at the moment but it's better to be safe than sorry.