I lived in New Zealand for 26 years and was a keen trout angler there and worked as a Freshwater Fisheries research technician for the government there. There are native trout in New Zealand (galaxias agentis) but they grow to only about 1.5 pounds. Early British settlers there introduced rainbow and brown trout from England during the 1800s, to all New Zealand streams, lakes, and rivers. Attempts were made to introduce lake trout from North America but these did not take very well and have survived as small lake-trout (2lbs max.) in only one small lake there.
I live in Canada and here a steel-head trout is a rainbow which has gone out to sea for a few years then returns to its river to spawn in freshwater. In New Zealand however, the rainbows do not do this, but the brown trout do, and they are called 'sea-run' browns and one of my friends there caught 27-pound sea-run brown which I saw.