AC88 asks "is it safe to say that this current counter bid could...

AC88 asks "is it safe to say that this current counter bid could not involve STO/COE/NHC/BPT/SXY as they are all currently trading?"

No. Not in all cases. ACN has a market cap of 132 million. So a bid from (say) Cooper Energy (COE) with a market cap of 201 million would be deemed significant for both companies, and both would be halted.

New Hope, on the other hand, has a market cap of 3.6 billion and Santos sits at 11.2 billion. A takeover bid by either of these giants for a pygmy like ACN would not be considered worth a trading halt - win, lose, or draw, the aquisition of a litte company like Acer Energy by the likes of Santos or New Hope would barely cause a ripple.

For an example of this, consider Pattinson's (SOL, 3.2 billion) recent bid for Exco (EXS, 97 million). EXS went into a halt while it was being negotiated, SOL carried on trading without a pause.

What about Beach (1.75 billion) and Senex (875 million)? I don't know where you draw the line. At a guess (pure guess!) you'd maybe expect Senex to halt (it's only about 7 times bigger than ACN) but Beach to trade on (it's about 15 times the size of ACN). Perhaps there is a formal guideline somewhere, but most likely it's just a matter for judgment on the merits of the indivual case.