I believe that argument relates very specifically to the application of a particular law, section 89 of the Extradition Act 1999 (http://www.legislation.govt.nz/act/public/1999/0055/latest/D...), which sets out the procedure for 'things' that have been seized.
There are other applicable laws, however. Section 25 of the Mutual Assistance In Criminal Matters Act 1992 (http://www.legislation.govt.nz/act/public/1992/0086/latest/D...) says that requests from foreign countries for assistance in criminal matters should be made to, and decided by, the Attorney-General. In addition, in New Zealand, it is illegal under section 252 of the Crimes Act to access a computer system without authorisation. Under section 248 of the Crimes Act (http://www.legislation.govt.nz/act/public/1961/0043/latest/D...), authorisation includes legal authorisation, so the NZ Police can access seized hard drives under the terms of the search warrant, and a computer system includes a part such as a hard drive. If the FBI employees weren't authorised to do so under law yet, because the warrant was issued to the NZ Police, and they directly interacted with Kim Dotcom's hard drive without his permission, they could in theory be convicted here.
New Zealand law allows anyone to lay an information (i.e. file a criminal charge), not just police, but only the government can request that someone be extradited, and so it is unlikely that the FBI employees will ever be charged.
There are other applicable laws, however. Section 25 of the Mutual Assistance In Criminal Matters Act 1992 (http://www.legislation.govt.nz/act/public/1992/0086/latest/D...) says that requests from foreign countries for assistance in criminal matters should be made to, and decided by, the Attorney-General. In addition, in New Zealand, it is illegal under section 252 of the Crimes Act to access a computer system without authorisation. Under section 248 of the Crimes Act (http://www.legislation.govt.nz/act/public/1961/0043/latest/D...), authorisation includes legal authorisation, so the NZ Police can access seized hard drives under the terms of the search warrant, and a computer system includes a part such as a hard drive. If the FBI employees weren't authorised to do so under law yet, because the warrant was issued to the NZ Police, and they directly interacted with Kim Dotcom's hard drive without his permission, they could in theory be convicted here.
New Zealand law allows anyone to lay an information (i.e. file a criminal charge), not just police, but only the government can request that someone be extradited, and so it is unlikely that the FBI employees will ever be charged.
Disclaimer: IANAL