The only process INZ have at the moment for coming in across the border when someone doesn't have a visa with valid travel conditions is if they meet the exceptions to the border closure criteria (threshold is very high and most requests for an exception are turned down).
No word has come out about whether the Minister of Immigration will use his new 'powers' of the Immigration (Covid-19 Response) Amendment Act to put a blanket extension on all offshore visa-holder travel expiry dates. It was mentioned when the Bill was going through Parliament as what the amendments could possibly be used for, but nothing has been confirmed yet.