As director of the CIA, Panetta was responsible for the US's drone campaign in Pakistan, which has indiscriminately attacked the civilian population in pursuit of a handful of terrorists. As Phillip Crawford and David Henderson point out,
Under the Geneva Conventions and other international laws, it is a war crime to launch indiscriminate attacks affecting the civilian population or civilian objects with the knowledge that such attacks will cause excessive loss of life, injury to civilians, or damage to civilian objects. This distinction between combatants and noncombatants is fundamental to all humanitarian law.And that distinction is completely ignored by the indiscriminate policy Panetta developed and oversaw.
(There's also the matter of his organising the cover-up of the CIA's torture programme, which makes him part of a joint criminal enterprise, along with most of the Bush Administration, under international legal doctrines the US was instrumental in establishing)
New Zealand, like all civilised nations, claims universal jurisdiction for war crimes. War criminals can be arrested and prosecuted under the Geneva Conventions Act 1958. And that is exactly what should happen to Panetta the moment he steps off the plane.