@MrsSkylerWhite
Really, the idea that parents will suddenly need their adult children to care for them after decades of managing very well, thank you, is beyond patronising.
Don't assume you will not need help in your old age/elderly years...
If in turn they decide they want to move to Australia, we shall rejoice in their happiness and visit, often.
WOW. How wonderful for you that you have the money and time to visit Australia - and so often too. Most people are not so fortunate and will very likely never see their family member again if they move to Australia.
They owe us nothing and we owe them nothing.
Wow, what a hideously cold remark. 
I would be really upset if my adult DC fucked off to the other side of the planet, and they would if me and DH did. It will never happen though, as we love each other, and care about each other too much to to do this. I would only move away if I had no-one here who meant anything to me.
I do seriously question the motives and reasons behind people moving 100s and 100s of miles away from family and friends, and everything they know, and I definitely think it very odd when people move 1000s of miles away.
People will say 'work!' or 'career opportunities' .. or 'a shiny new life...' LOL, as if you can't get them in your own country. I do wonder what people are running away from to be honest. Coz they sure are running away from something. Or someone!
You don't move away - permanently - to another country, if you are happy where you are.