The tree in the pictures doesn't look sparse to me. Is that a current picture? Are you south or north of equator? Wondering if you are early summer or entering winter. That could affect leaf drop.

The blog address implies the OP is in Australia.

Australia yes, we are in our first month of summer.

Its thinning from the inside out, I tired to get some good pics but my camera refused to focus. here is one though

{{gwi:126406}}

Your tree could be a little short on nitrogen. But nectarine do naturally drop some healthy looking leaves. That would be the oldest leaves as is occurring on your tree. So I don't think this is serious.

ok thanks a lot