Is there issues with compatibility with buying games digitally from the US?
Not with mine.
Make sure you go to settings on your primary account (your Oz one) and activate the PS4 as your primary PS4 (who has multiple PS4s anyway...).
The way I work it is to go to Amazon US Store and just buy USD gift cards, redeem them and buy the games I want. I've bought Diablo3, Destiny and Child of Light. All of them without issue. And because you've made it primary ps4, all users on there can play all games.
So basically buy and download them under US PSN/SEN account, and play them under my Aust one.
Oh and only one account holder needs PS plus. So you can also sign up for PS plus from the US, about $60 cheaper per annum.