It is common. 'Hi ya' - the 'ya' originally having been 'you' - is the standard informal greeting in many parts of the north of England. In speech, the 'h' i s often dropped, so that it sounds like 'eye-ya'. I'm from the north of England, and I was an adult before anyone ever said 'Hi' to me. When I was growing up, the greeting was always 'Hiya', and never 'Hi'. I recently sent an email to a cousin of mine who still lives up there - I began my email 'Hi' and he responded with one beginning 'Hiya'. It's informal, but perfectly polite. If you walk into a shop in Manchester, you'll be greeted with a friendly 'Hiya'. If you're a man and a man is greeting you, he might say 'Eye-ya, mate'; if you're a woman, or a man being addressed by a woman, the greeting might be 'Eye-ya, love' (pronounced with the same vowel as 'book'). It would be seen as patronising to respond to 'Eye-ya, mate' with a formal 'Good morning, sir' - so you should respond with 'Hiya'.

Its an expression that is used in the US. Usually with close friends or when your in a rush and need to say something quickly.

It's very common here in New Zealand too.

Very casual. But I'm US based. I would spell it as one word, hiya, but it's so informal you wouldn't typically see it written except maybe in chat online.