Whether to sign up to the TPP is NZ's decision, but agreeing to it cedes power beyond just copyright, so withdrawing from it later is harder.

If NZ were to withdraw from the Berne Convention now, they would only suffer from the consequences in the realm of copyright. TPP appears to bundle several disparate things together, so there is serious leverage for others to prevent withdrawal if the treaty turns out to be significantly disadvantageous to NZ.