Multi Site.- Master Site Posts from sub sites

I am looking for a method of having any posts created on my multi-site – sub sites to also appear on the mast site.

So the posts page on the master site contains all posts made from all sub domains.

How would I go about achieving this.

I was hoping to do this in the main system code, but I may also consider doing some kind of export – import at scheduled times if it is not natively possibly

Regards, mccay_a