If Walker were the main buyer yesterday, he would now be over...

If Walker were the main buyer yesterday, he would now be over 20%.

Can someone tell me what the asic rules are? Is it correct that any party owning more than 20% has to come forward and make an offer,? Or are they simply obliged to announce to the market what their intentions are? I just spent about 30 minutes looking for the regulations and couldn't find them. After many years of trading I should know these things by now!!