Andrew Leach discusses the mechanics of Canada's withdrawal from Kyoto. In short, if Canada gives notice of Kyoto withdrawal before year-end, it suffers no penalties from current non-compliance. To become compliant, Canada would need to spend perhaps $19 billion on international carbon credits. But the only penalty for non-compliance relevant to Canada, even if they stay in, is a harsher target in a second period regime from which they're abstaining anyway.
Meanwhile, we're getting set to sign on for the second Kyoto period, if any deal goes forward, though Tim Groser seems less keen on any deal that doesn't include the US or major developing-country emitters.
I've noted before that NZ's excess emissions are only a cost on the public purse if we choose for them so to be; if there's no second Kyoto period, there may be no point in buying up credits for first-period non-compliance.
But there's one consideration here relevant that isn't for Canada: non-compliance also keeps us out of the international market for selling carbon credits. And that would hit forestry guys selling credits abroad. And so there could be distributional issues domestically if New Zealand were to be non-compliant.
Disclaimer, as always: I reckon buying insurance against bad case global warming worth the investment. But I'm not convinced that New Zealand's ETS is at all worthwhile unless it's part of a global agreement for emission abatement. Absent that, I still reckon we do better by making higher variance plays on technology development.