New Zealand has been an inhabited island for thousands of years, but perhaps the most known aboriginals are called the Māori. The Māoris were polynesians, who migrated towards New Zealand and over a period of time developed their own language and cultural practises. The first settlers are thought to have landed somewhere around 1300 CE.
"Kia Ora" is an expression in the Māori language, and simple means "hello". Literally translated it has a meaning of "be well or healthy". These days you see and hear "kia ora" all over the New Zealand, and it is considered one of the most important words every kiwi should know. The Māori from the Pacific Island Cook have a similar word "kia orana" with an equal meaning.
The introduction of Maori words and phrases is relatively a new practive in New Zealand. In fact, the Māori words were considered of "lesser value", and this mindset only started changing from mid 1980's onwards. These days you see the cheerful expression everywhere, even the national airline, Air New Zealand, uses the word as the name for their inflight magazine.