transaction in shares, page-4

All other things being equal (and assuming that the market prices BHP reasonably accurately - no reason to believe that it doesnt) the value of BHP's remaining shares increase in value in proportion to the number of shares being bought back and cancelled.

BHP actually do better than that, beacuse they target :

1) on-market buybacks in London, where BHP normally trades at a discount to the Australian listed shares

2) off-market buybacks in Australia, where the bid system achieves below market buyback prices (the reason being that the ATO has ruled that a large proportion of the buyback price is deemed as a fully franked dividend, which makes this a good deal for institutions which typically pay tax at 15%, half of the 30% imputed tax carried by the dividend)