Missed this one at the time.
Re: feral pigs, I have some suspicions that their absence from New Zealand may have been deliberate rather than coincidence. I read an article a while back (I linked to it somewhere at T'Other Place years ago, but can't remember where) that some Pacific islands where pigs weren't available, the locals there knew about them and didn't want them, because of the damage they caused. So possibly pigs' absence was deliberate. On the other hand, chickens weren't present either, and it's hard to see why people wouldn't want them if they had access to them.
Either way, going with the assumption that porkers and chooks show up, yes, first implication is that the population of Aotearoa will be somewhat higher. Not hugely higher, mind; domesticated animals don't provide as many additional calories as better staple crops would. (Though I yam not sure what the best alternative staple crop might be.) But still, everything helps. More convenient sources of protein will also help the overall health of the population a bit, too.
I don't think it would make much difference in the South Island, though, particularly in the more southerly portions. Being able to eat pig and chook every few weeks doesn't make up for not being able to eat a staple crop every day. The crops that were available in the South Island (cabbage tree, bracken fern) weren't useless, but couldn't support a high population.
Butterflies aside, it's also not going to help the Maori keep a majority in Aotearoa. That would take something which can allow the Maori population to increase by something like four-fold, which isn't going to happen without some seriously good staple crops. (Wattle they do to find such things?).