NZ Post says there may not be an efficient way to ensure GST is consistently paid on overseas internet shopping purchases before 2020.

NZ Post may charge people $20 to receive overseas parcels if the Government slashes the threshold under which items can be bought from foreign websites GST-free.

The new fee of between $15 and $20 per parcel would cover the cost of red tape associated with collecting tax at the border, NZ Post said in a document obtained under the Official Information Act.

People could also expect more delays receiving parcels from overseas, it said.

Customs Minister Nicky Wagner is due to release a consultation document in April that will canvass lowering the $400 threshold under which most overseas internet shopping purchases can be made free of GST and duty.

Retail lobby group Retail NZ has been pushing for the threshold to be slashed or abolished, to put domestic retailers and overseas sellers on a level playing-field paying GST.

But NZ Post said that if the threshold was slashed from $400 to $20 and tax checks continued to take place at the border, then it would need to invest $20 million in a brand new parcel warehouse that would also cost several million dollars a year to operate.

NZ Post said it already incurred costs of $700,000 a year helping facilitate the payment of GST and duties on parcels entering New Zealand.

But if the threshold was slashed it would not be able to absorb the increased costs, it said.

"Based on our preliminary analysis, New Zealand Post's cost recovery charge is likely to be in the range of $15 to $20 per parcel held at the border for GST collection," it said.

That meant consumers might expect to pay $65 in administrative charges on low-value items they bought from overseas if an existing $49.25 Customs clearance and biosecurity fee also applied to all items under any new threshold, it said.

NZ Post made the warning in a submission on a separate law change that will see foreign sellers forced to levy GST on sales of digital services to New Zealanders from October next year.

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The tax on digital services is commonly referred to as a "Netflix tax" while the lower threshold on physical imports that prompted NZ Post's warning is often referred to as an "Amazon tax".

NZ Post said the costs it would face from an Amazon tax would depend on whether GST and duty continued to be collected on parcels as they entered New Zealand, which is the situation at present.

Its costs could be reduced if the Amazon tax was collected in the same way as the planned Netflix tax, through a requirement on foreign firms to register and collect GST on behalf of the taxman.

However, the Digital Economy Group, an informal coalition of major e-commerce companies that is believed to include Apple and Amazon, signalled its opposition to the latter idea in its submission on the proposed Netflix tax, also released under the Official Information Act.

If the GST threshold on physical items was reduced or eliminated, GST should be collected by "New Zealand authorities" and not by foreign sellers, the coalition said.

NZ Post said it envisaged the Government might introduce a "combination of GST collection mechanisms" if it moved to collect GST on more imported physical items.

But even if foreign sellers were held responsible for levying GST on all imported items, NZ Post would still have to invest in new parcel sorting technology and software to detect items for which GST had not been prepaid, it said.

Canada Post was currently piloting such a "data exchange", which was being developed by the Universal Postal Union, but NZ Post said it didn't expect "global uptake" of that solution before 2020.

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