oops .. make that Part 4 (ss81 to 89) of the Tax Administration Act ...

It doesn't legitimise using private data to attack political opponents. But if that *is* fair game, can I put an OIA request in for John Key's tax return?

Of course you can.

Not only that, but there's nothing in the Privacy Act will stop them releasing it to you.

Yes, but I can't imagine any Dept releasing that, given that privacy is a ground for withholding under section 9(2)(a) of the OIA.

I think Part 4 (2281 to 89) of the Tax Administration Act 1994 trumps the Privacy Act and the Official Information Act. It sets out the extremely limited circumstances in which tax information may be release, to whom and for what reason.
Basically a taxpayers rights to secrecy overules most things.