A $NZ2 billion ($1.91 billion) takeover offer by Ampol for Kiwi petrol and diesel retailer Z Energy has left some investors unconvinced they will benefit from the deal, after the offer price was raised three times amid the possibility rival suitors may still be sniffing around.

Z Energy chief executive Mike Bennetts is not ruling out the possibility of a rival bidder still emerging, even though the New Zealand company cannot solicit alternative approaches after granting exclusive due diligence to Ampol with a view to firming up a transaction.