Green’s Dictionary of Slang

pie adj.2

[Maori pai, good]

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

[Aus]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.
[NZ]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.’.
[NZ]McGill Reed Dict. of N.Z. Sl. [as cit. 1988].