Thank you for submitting your item for appraisal. This is a most interesting item which appears to be man made. I think your assumption is right that it is lead, and covered with a thick layer of lead oxide / authentic patina. The shape is that of a western style axe. But a metal aboriginal axe would be truely unique. Prior to cultural protection laws stone axes sold in the low hundreds if at all. You might find it difficult to sell it at all, as you are not able to establish a chain of ownerships / provenance.
I would send detailed information of your find to the National Museum of Australia and accept whatever they offer. You might gain your fifteen minutes of fame.