Please see the below notice from NZ Post regarding disruptions to the courier network:
Unfortunately, an unforeseen issue caused a major disruption to our aircraft fleet on Monday that reduced our operating schedule to a single aircraft. 

We are working as quickly as possible with our airline partner to reinstate our full air freight capacity and expect this work to be completed in the next few days.
We have already taken steps to divert items to alternative transport routes to get them to their destinations as quickly as possible, including; 

• Alternative air transport via Air New Zealand where space allows for time-critical items 
• And alternative road routes for the majority of courier items 

However, the nature of this disruption means that it is likely to cause some delivery delays to courier items moving between the North and South Island over the remainder of the week. 

Our contingency plans are also taking into account notice received yesterday of unplanned maintenance to one of the Interislander ferries, as well as road closures that remain in place following the severe weather over the last few weeks.
We are working hard to ensure parcels reach their destinations as quickly as possible and will continue to update our Domestic Delivery page online with our latest delivery timeframes. Please only call our Customer Contact Centre for business critical enquiries – for everything else check out our online tools including parcel tracking. Thank you for your support and patience as we work through this together.