Green’s Dictionary of Slang

(N.Z.) good at, expert in.

1941		Baker N.Z. Sl. 56: To be pie at (or on), to be expert or efficient at something, is another phrase of wide use in this country. It has been drived from the Maori pai, good.
1988		McGill Dict. of Kiwi Sl. 83/2: pie good, pie on expert or efficient, good at, from Maori ‘pai’, good, ‘pai ana’, suitable; eg ‘That paint job was pie on, ta muchly.’.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].