The GST would be levied on foreign firms. It is expected to yield about $64 million in its first year, rising to $81 million in two years with growth in online shopping.
The Herald report said that many people would see the extension as a broken promise by Labour, which had committed to no new taxes in its first term.
Ardern told Newstalk ZB's Mike Hosking that the policy, to be formally announced at 11.30am today, would be an extension of GST.
Before the last election, Labour promised not to implement tax changes this term arising from the tax working group.
But applying GST to low-value goods was specifically ring-fenced from that promise by Labour's revenue spokesman at the time, Michael Wood.
The process was already being developed in the system by IRD under the National-led Government.
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 items such as a $40 book means it is not collected.
Today's announcement is likely to find a way to make large sellers comply with New Zealand law by registering and collecting GST.
While the tax will be paid by the foreign firms selling the goods the cost would inevitably be passed on to the buyer in the form of increased prices.
