One of the core concerns about the GCSB is that they don't really work for us, but for the US. As part of this, they host NSA staff, not as arm's length liaisons, but completely integrated into their organisation. We've known about this since Secret Power, and the former Deputy Director of the NSA this month admitted it on NZ TV:
"There is a sharing of personnel," says Mr Inglis. "We call them 'intergrees' across the Five Eyes partnership."
So there is no reliable proof of bases, but the NSA does have staff working in New Zealand.
However, when the GCSB is asked directly via the OIA whether they host NSA staff, they refuse to answer:
In response to your request, under the provisions of section 10 of the Official Information Act (the Act), I can neither confirm nor deny the presence of a Special US Liaison Officer in New Zealand or a New Zealand Liaison Officer in the United States, on the grounds that disclosure of this information would prejudice the interests protected by section 6(a) of the Act.
This fools no-one. Firstly, as noted above, its a documented fact. Secondly, given the optics with the New Zealand public, if there weren't such NSA plants in the GCSB, they'd surely admit it. So their "refusal to confirm or deny" can really only be taken as an admission of guilt.