Can I change my mind after making a successful bid?

You cannot change your mind after winning a property auction without consequences.
When the hammer falls in an unconditional auction, it represents an exchange of a binding contract between the seller and the buyer. It is too late for either party to change their minds, and the sale is required to proceed in accordance with the contractual terms and at the price that was concluded when the hammer fell. There are penalties for buyers or sellers if they're the reason a sale cannot be completed.

Anyone bidding in an unconditional auction needs to remember that the winning bidder exchanges contracts straight away, and if you are unsure about buying the property at any time, then you are advised not to bid.

When the hammer falls in a conditional auction, the buyer and seller enter into an exclusivity agreement, and the successful buyer pays a non-refundable reservation fee. If the buyer fails to complete the sale in the agreed time period, then they would forfeit the reservation fee and could be responsible for additional fees.

Can I renegotiate my winning bid after the auction?
No, you cannot renegotiate your winning bid after the auction, this is called gazundering. All bidders are advised not to bid beyond their maximum amount for this reason.