What is this nut tree?

This small tree grows along roadsides close to the bank of the Colo river west of Sydney.
It has fruits containing nuts.
The internal structure of the fruit looks something like a mangosteen, but the thin pulp has an astringent flavour.
Inside the the thin shell is a pure white nut. It tastes pleasantly mealy, but not delicious. I spat it out in case it is poisonous.
I don't think it is a native Australian species and there are a lot of garden escapes in this area.
What species is it?

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.