My lady explained it as meaning pretty much the same as the English version - 'Oi' Just their way of pronouncing it. And as already said, as even in English, that little word can be used in times of frustration, joy, excitement, upset etc..
She could be totally wrong and have made it all up - but is good enough for me - besides. The way the girls say it - whatever they're trying to shout about, is so darn cute - it always comes across as funny
UUuuaaeeee...........