a new documentary hopes to expose the illegal organ trade in china .
china 's organ trade is allegedly worth a massive us$ 1 billion a year .
human rights lawyer david matas tells the gory details of what goes on .
tens of thousands of innocent people killed on demand it is claimed .
political prisoners in particular being used as live organ donors .
it 's believed most were members of the banned falun gong movement .
documentary will air on sbs one 's dateline program on tuesday night .