The NZeTA also became mandatory for visitors from 60 visa waiver countries on October 1. It must be applied for before a visitor enters New Zealand and costs $9 on INZ's mobile app or $12 on its website.
National's Tourism spokesman Todd McClay said people were turning to the unofficial apps and websites because the official ones were too difficult to use.
"Kelvin Davis [Minister of Tourism] promised to make it easier for people to visit NZ, but his rushed implementation of tourist tax has instead led to unofficial websites and a clunky app," McClay said.
"The Government has released an official app, but each visitor has to use it separately so families cannot pay the tax in one simple transaction. It's bureaucratic and a hassle for travellers."
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McClay said MBIE was correcting up to a thousand applications each week because of the app's poor reading of passport numbers and names.
"The Government was warned its new tax would mean fewer visitors and a loss of up to $70 million for small businesses...we're already seeing the consequences...with falling numbers from major markets like India and China," McClay said.
INZ said a growing number of companies that had no connection with INZ or the NZ Government are operating under potentially misleading names such as "official-newzealand-eta.org" and "newzealand-eta.gov.org" were charging visitors up to 10 times the official price.
Stephen Dunstan, INZ general manager, said: "We are seeing a growing number of third parties charging visitors for something they can very quickly and easily do themselves and that is concerning."
Dunstan said the application took just a matter of minutes to complete on the agency's official sites.
"Some third parties are also claiming to be able to provide an expedited service. However, an expedited service is not available and third parties are only able to submit the traveller's information to INZ using the same web form or mobile app travellers can use themselves," he said.
"In some cases, these third parties are also collecting additional information from travellers, which isn't actually required as part of the NZeTA request."
The agency, Dunstan said, was working hard to ensure travellers knew to use the official government website through its global marketing campaign.
"Our global digital marketing campaign, which links directly to the INZ website, is helping to ensure travellers use the official channels to request their NZeTA.
"We're also working closely with airlines, travel agents and cruise operators to make sure they're directing travellers to the official government website or mobile app."
