Revenue Minister Stuart Nash said requiring overseas internet sites to levy GST was "only fair' when he announced the tax change in 2018.

Amazon has warned it could cut the range of products it sells to New Zealanders if the Government presses ahead with its plan to make foreign firms levy GST on low-value goods shipped to New Zealand.

The tax change – which is scheduled to kick-in from October next year – would change the situation where most goods under $400 can be bought from overseas tax-free and has been welcomed by local retailers demanding a "level playing field" with foreign online stores.

But Amazon said in a submission released under the Official Information Act that it was concerned the approach was "unenforceable and unworkable".

Amazon would need to assess the feasibility of the plan once it was clear how it would be implemented, but could need two years to prepare, it said.

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"This assessment may result in changes to the products and services Amazon is able to provide to NZ consumers," it warned.

The company has already demonstrated that may not be a hollow threat.

Australia became the first country in the world to require foreign firms to levy GST on international deliveries in July.

But Amazon announced in May that it would instead block Australians from buying from Amazon.com, while continuing to let them buy from its smaller Australian service, Amazon.com.au, which is stocked from a warehouse in Melbourne.

While the strength of Amazon's opposition to the so-called "Amazon tax" is unlikely to surprise or rattle ministers, the country's peak business body BusinessNZ also appeared to be wobbling over the practicality of levying GST on overseas internet shopping.

LEON NEAL/GETTY IMAGES The growth of online shopping has made the GST issue impossible to ignore, but so may be the difficulties in tacking the issue.

​BusinessNZ had appeared to line-up with local retailers in favour of an Amazon tax when Revenue Minister Stuart Nash announced in May that New Zealand would follow Australia by closing the GST loophole.

The peak business body has told Inland Revenue in its submission that it still supported the idea "in principle", but said the developments in Australia showed it "might not be as straightforward as first believed".

BusinessNZ said Inland Revenue should assess whether other large foreign firms would pay ball with the Amazon tax in Australia before proceeding, and should delay the tax change if there were "ongoing persistent concerns" about overseas suppliers cutting off Kiwi consumers.

The interests of businesses, consumers and the Government needed to be balanced, it said.

"If some of the largest suppliers to the New Zealand market are blocking customers, then ... the benefit of revenue collection may be superseded by loss of consumer choice."

A spokeswoman for Nash said the decision to push ahead with an Amazon tax had been made and there was no u-turn.

Officials are due to provide Nash with advice on the design of the legislation in about two weeks, she said.

The Government has already factored in an extra $213m in revenue over four years from the tax change, in the May Budget.

KEVIN STENT/STUFF National revenue spokesman Paul Goldsmith says the Government needs to be "open minded".

Retail NZ spokesman Greg Harford said Amazon's stance was no surprise but it was disappointed by BusinessNZ's comments.

"There is no compelling reason to delay beyond October next year."

The association believed most large foreign firms were complying with the new regime in Australia, including British book seller Book Depository which has been owned by Amazon since 2011, he said.

"It is very clear to us that if Amazon wanted to comply, it could."

National revenue spokesman Paul Goldsmith said he understood retailers' sentiment and agreed with the notion of an even-playing field.

But he believed the Government would need to be "open minded" about adapting its policy and potentially pushing back the implementation date, depending on what it learnt from Australia.

"I don't think you can afford to be totally dismissive of the broader issue that it would be a bad outcome if New Zealand consumers lost access to those global stores – that is not an outcome that we want."

Amazon is not the only e-commerce firm warning that it might not be ready to charge GST on international shipments by October next year.

SEAN GALLUP/GETTY IMAGES Amazon fears the tables could turn with only large foreign companies forced to levy GST and smaller firms escaping the net.

A joint submission from online marketplaces eBay, Alibaba and Etsy said they would "strive to make the investments required" to collect the tax but said it was wrong to assume that would be simple.

New Zealand would be only the second country to go down the path and there was a "high risk" consumers and businesses would face complications because its approach was "novel, untested and complex", the three firms said.

The 180 pages of submissions released by Inland Revenue set out a wide range of detailed complications that could arise implementing the tax, as well as strong support for the change from Retail NZ and Booksellers NZ.

The broader concern voiced by many submitters was that few foreign firms might comply, and that Kiwi consumers might then be more likely to shop with those that didn't, exacerbating that problem.

Amazon said an exemption for foreign firms that sold less than $60,000 of goods to New Zealand each year would make it "nearly impossible for the Government to detect noncompliance for all but the largest offshore suppliers".

Consultant EY supported the proposed changes in principle but also questioned Inland Revenue's assumption that most foreign firms would play ball.

"For all but the biggest suppliers, the collection of GST will in substance be voluntary," it said.

JARRED WILLIAMSON/STUFF Revenue Minister Stuart Nash is expecting advice from Inland Revenue on how to implement an "Amazon tax" later this month, amid no signs of a backdown.

The documents suggested Trade Me – which is one local business that stood to lose out from the Government's original proposals – might have won a key concession.

It was concerned that it would be forced to levy GST on all goods sold by foreign traders to Kiwis through its website.

But Trade Me indicated in its submission that Inland Revenue had issued a "memo" subsequent to the release of an original discussion document that signalled New Zealand-based online marketplaces would be excluded from that obligation.

That would appear to mean Trade Me's larger foreign traders would levy the tax themselves, and only if they individually crossed the $60,000 annual sales threshold.

The Corporate Taxpayers Group, which represents 41 of the country's largest companies, backed the thrust of the Government's plan but argued the $400 threshold below which foreign firms should be obliged to collect GST should be raised to $1000.

Items above the threshold would continue to have GST, tariffs and biosecurity fees applied at the border.