The Prime Minister says the introduction of GST on online overseas goods is about fairness for retailers, rather than revenue for the Government.
Jacinda Ardern says New Zealand-based retailers want an even playing field where all products, no matter where they come from, attract GST.
She says it won't apply to overseas businesses which supply less than $60,000 worth of product.
Ardern told Mike Hosking it will bring money into the Government's coffers.
"That wasn't a driving force. For me, a big motivation is simply, you know, when you've got New Zealand retailers saying all we want is fairness."
Revenue Minister Stuart Nash will make the announcement at 11:30.
GST is payable on all goods bought in New Zealand, including online purchases such as books bought on Amazon or Book Depository or goods from Alibaba and e-Bay.
But the cost of collecting it on small things such as a $40 book means it is not collected.
Last year Nash said the National Government had not gone far enough when it introduced the so-called "Netflix tax" to gather GST on services and electronic purchases.
"It gives a 15 per cent competitive advantage to [retailers] overseas. While our retailers had to deal with GST, overseas people didn't. It's the right thing to do."
The tax being announced today had been planned by National with Judith Collins sending a paper on it to Cabinet in July last year.
The "Netflix tax" kicked in October of 2016, requiring sellers such as Apple to charge GST on online purchases from New Zealand.
Despite that, today's announcement would be seen by many as another broken promise by Labour which committed to no new taxes in its first term.
It came after a review in 2015 found the Government was missing out on about $180 million a year by not collecting GST on online purchases, including $40m from shopping on iTunes, Netflix, Spotify and other online services.
Nash's move was heralded as "the first time that a Revenue Minister has committed to fixing this issue" by Retail NZ spokesman Greg Hardford.
Local retailers had long been at a disadvantage to overseas sellers, Hardford said.
However, that did not impact on purchases of goods from overseas which were worth less than $400 until now.
The same tax will be introduced to Australia in July.
LISTEN TO JACINDA ARDERN TALK WITH MIKE HOSKING ABOVE