last 'top twenty ' list of shareholders, page-25

Steini,

No reason why not. As long as they stay within any foreign ownership restrictions etc then they can't buy up as much as they wish. The institution is responsible for monitoring the underlying positions which I believe are generally declared to the company via substancial holding notices. AU law would still apply to all beneficial owners even if they hold ADRs so once they get over the 19.9% mark they will need to make an offer as you point out.

Not sure any of this is anything to worry about. If the price isn't what you want just don't sell.