Kia ora is an informal greeting, like "hi" or "gidday" in English and is used to greet friends or to say hello in a casual way.
However, if you are meeting someone important, or you want to greet someone formally, then you would use a more formal greeting:
Tēnā koe.
There you are.
Hello
The word tēnā means, "there, by you". So tēnā koe literally means "there you are".
It is a little bit like the Na'vi in Avatar who greet each other by saying "I see you". It is a polite way of acknowedging the other person.
A long time ago there was a plural word in English for "you" - it was "ye". Sometimes people today say "yous" as a plural form of you ("can I come woth yous guys?"), but mostly we no longer differentiate between talking to one person or many people.We just say "you".
Te reo Mōai,however, does have a different word for "you" when talking to one person (koe) and talking to 3 or more people. This word is koutou.
You have probably heard it used:
Tēnā koutou.
There you (all) are.
Hello.
This is a polite form of address to three or more people.
Another word that can be used is katoa, meaning "all". And you have probably heard this used in speeches:
Tēnā koutou.
Tēnā koutou.
Tēnā koutout katoa.
There you are.
There you are.
There you all are.