Its largely what you'd expect: Fonterra with all its cows, petrol companies, the two electricity companies still clinging to fossil generation, Air New Zealand, and a few large industrial emitters. Not included, because they fell just outside the top ten: the Tiwai Point aluminium smelter, which demanded a fat emissions subsidy from the government, then halved its emissions, and Bluescope (formerly New Zealand) Steel, which runs a dirtier-than-average steel factory in Glenbrook.
Stuff had to dig this information out of companies' annual reports, because currently it is all legally secret. The good news is that that is going to change: the government has decided to publish firm-level ETS data, allowing the public to scrutinise high-emitting companies and vote with their wallets where they can. Which suggests that there's absolutely no need for the secrecy clause anymore, and that it should be repealed entirely.