The Customs Service is training postal staff to screen incoming international mail for items liable for GST and duties, saying the move will free up its officers to spend more time detecting contraband.
Kiwis' love of retail websites such as amazon.com means increasing amounts of goods bought online are coming across the border.
Freight operator DHL Express said last month that it had seen a 30 per cent rise in its inbound deliveries during the first quarter this year - primarily as a result of online shopping.
Most items bought on overseas websites can be imported free of charges if they are valued at less than $400, including freight and insurance costs.
However, the threshold is lower for goods to which duties apply, such as footwear and apparel.
A Customs spokesman said New Zealand Post staff were progressively taking over the screening function at Auckland's International Mail Centre and would continue to be supported by Customs officers.
"NZ Post was already invoicing for duty and GST so identifying the items to invoice was simply an extension of this work," he said. "There will be two [NZ Post] people screening when full implementation is complete."
The New Zealand Retailers Association has been calling for all items bought on overseas websites to be subject to GST, which the industry group says would result in a more level playing field for local businesses.
About 34 per cent, or $910 million, of Kiwis' online purchases are thought to have been made on overseas websites last year.