Whether New Zealand can extradite Kim Dotcom and his Megaupload former colleagues for something that is not a crime here is the centre of today's Supreme Court hearing.

A mammoth legal fight over the extradition, spanning seven years since Dotcom was arrested in 2012, has reached its final appeal in Wellington.

Starting on Monday last week, the Supreme Court appeal heard from lawyers for Dotcom and his alleged co-offenders Mathias Ortmann, Bram van der Kolk and Finn Batato. All have argued against extradition.

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The legal arguments are expected to come to a close this afternoon or tomorrow morning.

Crown lawyer for the US, David Boldt, today focused his arguments on New Zealand's Extradition Act 1999 and the extradition treaty with the US.

He said where the act and the treaty differed, the treaty should be followed.

The argument centres around issues of whether copyright infringement amounts to a criminal offence in New Zealand, and therefore an extradition offence.

If it does not, a process called double criminality would apply - meaning for extradition to be successful, the copyright infringement would need to be considered a crime in both the US and New Zealand, rather than just in the US.

Double criminality was overturned in a Court of Appeal decision, United States of America v Cullinane, a case from the early 2000s in which Waikato man Bob Cullinane was accused of being part of a US$3.7 million ($8.8m) truck driver smuggling scam.

But when the Megaupload case reached the Court of Appeal, double criminality was reinstated.

Boldt is again tackling that issue, saying double criminality does not need to apply in this case, because the wording of the act implied the treaty should trump it.

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"The purpose of the act is to ensure the law of the land conforms to the treaty," he said.

But lawyer for Ortmann, van der Kolk and Batato, Amanda Hyde, said the treaty limited what people could be extradited for, rather than expanded it.

"The treaty itself defers to domestic law," she said.

Dotcom, Ortmann, van der Kolk and Batato were found eligible for extradition to the US in 2015 to face charges relating to massive copyright abuse which cost movie and music producers US$500m ($750m).

The FBI alleges the men ran the website Megaupload to profit from pirated movies and music downloaded by users from the site.

The legal battle over Kim Dotcom's extradition has raged for seven years. Photo / File
The legal battle over Kim Dotcom's extradition has raged for seven years. Photo / File

Megaupload was taken down in 2012 on the same day the group was arrested.

They deny the allegations, saying Megaupload was simply a service offering cloud storage to users, and claim copyright breaches were policed in accordance with the law.

Appeals against the extradition have gone to the High Court and Court of Appeal and have failed both times. The current hearing in the Supreme Court is the final appeal on the matter - though not the group's last legal avenue.

The next step, if the court is to allow extradition, is for Minister of Justice Andrew Little to decide whether to sign extradition paperwork. This could then be subject to a request for judicial review, and could go on to prompt further appeals.

The men face charges of racketeering, money laundering and copyright infringement.