what's the international phone code for australia calling from hong kong?

Where in Australia are you dialling to?

If you're dialling to Sydney: 001 61 + number

Codes are the same for each country no matter where you are.
Australia is 61.
If it is on a cell phone, then you just add a + . Cell to cell. They pick it up. No need for area codes extra.
But for land lines, 00-61-area code- .....
In all probability you have the are code with the #. So just try the 61+#

061....this is the ISD code for australia...

ISD codes dont change.... its same--not depepnded on where you are calling from, but to where you are calling to...

I pretty sure Australia's country code doesn't change no matter where you call from.

it will be thesame calling from anywhere else but i don't know it!

your call out number then push 61

+61 - not sure about hong kong but form many countries this would be 0061, then the area code (8 for western australia, where I am), then the number

good luck