Section 611 of the Corporations act allows for various...

Section 611 of the Corporations act allows for various exemptions to the takeover provisions.
One of the exceptions is:

A Downstream acquisitions that results from acquisition of another company is exempt.

So ,company A owns shares in company B.
Company C also owns shares in company B.
Company A can buy out company C and by default will acquire more shares in company B.
This acquisition is exempt from takeover triggers.

I suppose it is designed not to disadvantage company C in the mechanics of possible takeover bids.