At the time the van had 195 items on board waiting to be delivered around Auckland.
The theft was reported to police, who found the van abandoned 2km away later the same day.
CourierPost spokesman Aaron Hanson said 75 items were missing from the van, but the rest were still inside.
One man, who did not want to be named, told the Herald he had lost about $250 worth of goods.
"I bought two items online on March 30 from an outdoor sports equipment shop in Britain - a pair of waterproof pants and an inflatable camping mattress together worth about $270 including delivery," he said.
"When they didn't turn up after 10 days or so I got the Royal Mail track and trace number from the shop. Using this I learned that the package had arrived in Auckland and was being 'processed'."
Yesterday he contacted NZ Post and was told that the parcel had been stolen from a CourierPost van.
"He said an April 4 'scan' of my parcel stated that it was 'out for delivery'. He checked the CourierPost website and in a posting at 11.23am on April 5, CourierPost had said the item was stolen."
The man then spoke to CourierPost, which confirmed his parcel was among those missing.
"I'm annoyed that CourierPost or NZ Post didn't contact me to tell me my goods had been stolen. If they had done so, I would have stopped wondering each day whether the parcel would arrive," he said.
"More importantly, I would have immediately begun looking on Trade Me to see if the thieves were trying to sell my items."
The man had contacted the British store by email, explaining the situation and asking if they would refund him or send replacement items. He said he had still to hear back from them.
Mr Hanson said CourierPost had managed to contact the bulk of the senders within New Zealand, advising them of the theft so they could initiate the claims process.
"We only have access to the domestic sender's details as this is who has purchased the service directly with us.
"In the case of international items the situation is different as we are delivering the item on behalf of NZ Post International from an overseas mail or courier business and therefore do not have any of the sender's details so we cannot make the same contact," he said.
"In this situation we provide NZ Post with a list of those items affected so that they may follow up with the sender as well as flagging the tracking number in our system should the receiver contact us following which we can instruct them on what has occurred and who to contact in order to make a claim.
"Unless a manifest is provided by a sender, we do not hold the receiver's details so it is not possible to contact those to whom the items are being sent."
Mr Hanson said all affected customers had been advised of the claims process they had to use so they could be compensated for their loss, which he described as "unfortunate".
"Thankfully these types of incidents do not occur often and our couriers take every measure to keep their freight safe, however it is not possible to guard 100 per cent against those who are intent on breaking the law."