Critics of new copyright rules under the Trans Pacific Partnership (TPP) say they will cost consumers millions of dollars and stifle the creative industry.
The government said the deal between 12 Asia-Pacific countries would inject at least $2.7 billion a year into New Zealand's economy by 2030.
In a statement, it said the only significant cost came from extending New Zealand's copyright period from 50 to 70 years.
"This cost - in terms of foregone savings on books, films, music and other works - increases gradually over 20 years and averages around $55 million a year over the very long term."
Lawyers and creative industry leaders said the copyright extension was a trade off and there were mixed benefits for New Zealand artists.
InternetNZ's chief executive Jordan Carter said one of the biggest winners was Disney with its Mickey Mouse character.
"People who own copyrighted material that was about to expire, and they would lose the royalties from that will keep those royalties for another 20 years. So it benefits people who've got famous pieces of intellectual property from the 1930s and '40s; Mickey Mouse owned by the Disney Corporation is a classic example."
Mr Carter said the extension meant consumers would have to pay for longer and New Zealand artists would miss out.
"A lot of what people do today in the creative sector is remix or reuse. They take old concepts, they reinterpret them for the current day and when something's in the public domain, when it's hit the end of its copyright term, there's no uncertainty about your ability to do that," he said.
"But when it's still in copyright, you have to try and find out who the owner is, you have to try and find out how much you need to pay, there's less raw material out there for people to riff off and be creative."
Auckland University Press director Sam Elworthy said the copyright extension would not matter much to authors.
"Fifty years is a long time, 70 years is a bit longer and by that point most books have reached the end of their useful life, or their sales life anyway."
But, he said, it could make a big difference to other artists like singer Lorde because the copyright is from the date of the recording.
"It means that Ella Yellich O'Connor will be enjoying royalties from 'Royals' in her old age when she wouldn't have beforehand."
Simpson Grierson intellectual property lawyer Earl Gray said many believed most of the money earned from the extension would go offshore.
"The beneficiaries are those who are the most prolific authors who make the most sales. Some say that that will be the publishers who control those rights. I suppose the general thought is that that will go offshore."
Mr Gray said New Zealand had traded off the extension for a better deal in other areas of the TPP.
"If we've got some significant trade benefits from having to do this I think it is one of the better things to be giving up."
Extra charges to hit universities
Auckland University copyright officer Melanie Johnson said the extra charges would hit universities but they would not be able to pass them on to students.
However, she said there was a much bigger cost.
"A public domain populated with New Zealand works is going to be locked up for another 20 years. So at the moment we are only just starting to get, for example, TV shows that were produced in the 1960s [where] there's a copyright or will shortly come out of copyright.
"Universities could have made free use of those if they were in the public domain, and the wider public could have made use of those works."
The Publishers' Association said it was wrong to say the extension locked up New Zealand works because copyright material could be easily accessed.
And its vice president, Mark Sayes, had no sympathy for the universities having to pay more.
"Why shouldn't they? In the end, if they want to use the hard work, the graft from authors for an extra 20 years, then they can expect to pay a reasonable fee for it."
Mr Sayes said the extension brought New Zealand into line with other countries.