That's an excellent question & had me searching the ATO website....

That's an excellent question & had me searching the ATO website. It depends on the details of the takeover offer, ie. scrip for scrip rollover or not.

In summary, if it is a scrip for scrip rollover offer, ie, shares for shares, then CGT will be deferred until the sale of the new shares. If it is not a rollover then CGT is payable.

I have been involved in one ASX takeover in the past & it was a cash offer as the entity was not listed. Started at 50% SP premium & there was a higher bid from another party & then a counter bid which resulted in 100% SP premium.

https://www.ato.gov.au/General/Capital-gains-tax/Shares,-units-and-similar-investments/Takeovers-and-mergers,-scrip-for-scrip-rollover/

Example: Scrip-for-scrip rollover

Stephanie owns ordinary shares in Reef Ltd. On 28 February 2020, she accepted a takeover offer from Starfish Ltd of one ordinary share and one preference share for each Reef Ltd share. The market value of the Starfish Ltd shares just after Stephanie acquired them was $20 for each ordinary share and $10 for each preference share.

The cost base of each Reef Ltd share just before Stephanie ceased to own them was $15.

The offer made by Starfish Ltd satisfied all the requirements for scrip-for-scrip rollover.

If the rollover didn't apply, Stephanie would have made a capital gain per share of:

$30 capital proceeds minus $15 cost base = $15 capital gain

The scrip-for-scrip rollover allows Stephanie to disregard the capital gain. The cost base of the Starfish Ltd shares is the cost base of the Reef Ltd shares.

Note: Apportioning the cost base

As the exchange is one share in Reef Ltd for two shares in Starfish Ltd, the cost base of the Reef Ltd share needs to be apportioned between the ordinary share and the preference share.

Cost base of ordinary share:
$20 ÷ $30 × $15 = $10

Cost base of preference share:
$10 ÷ $30 × $15 = $5

Example: Takeover where shares were exchanged for shares with a cash proceed and the scrip-for-scrip rollover doesn't apply

In October 2010, Desiree bought 500 shares in DEF Ltd. These shares are currently worth $2 each. Their cost base is $1.50.

XYZ Ltd offers to acquire each share in DEF Ltd for one share in XYZ Ltd and 75 cents cash. The shares in XYZ Ltd are valued at $1.25 each. Accepting the offer, Desiree receives 500 shares in XYZ Ltd and $375 cash.

The capital proceeds received for each share in DEF Ltd is $2 ($1.25 market value of each XYZ Ltd share plus 75 cents cash). Therefore, as the cost base of each DEF Ltd share is $1.50, Desiree will make a capital gain of 50 cents ($2 − $1.50) on each share – a total of $250.

The cost base of the newly acquired XYZ Ltd shares is the market value of the shares in DEF Ltd ($2) less the cash amount received ($0.75); that is, $1.25 each or a total of $625 (500 × $1.25).