New Zealand Post is struggling under the weight of a record one million items a day and can't guarantee every Christmas parcel will make it under the tree in time.

LISTEN ABOVE: New Zealand Post chief operating officer Mark Stewart spoke to Tim Dower

Chief operating officer Mark Stewart said this is the busiest festive season it's ever faced.

He said they knew that would be the case and have been planning for it for a long time, but the volume of mail has exceeded even their most optimistic forecasts.

"To give you a sense of the planning that's gone into this, we have got  hundreds of additional vans and other vehicles, hundreds of additional people, a thousand additional truck journeys and we've even put on an additional Boeing airplane to cope.

But even with all that,  he says they are "stretched right to the edge."

Mr Stewart admits to some delays early this week but says they were working flat out last night and  will be again today and tomorrow to catch.

He said believes the overwhelming majority of Christmas presents and cards will be delivered by the end of tomorrow.

While he can't guarantee there won't be the odd slip up, Mr Stewart said staff are pulling out all stops to try to make it 100 percent.