BA's use POA's often at auctions not only for absentee purchasers but also to provide anonymity for the buyer up to the point of contract signing.
With a power of attorney, the attorney is acting "pursuant to a power of attorney they are acting" for, and on behalf of and in the name of, the Principal when it comes to making decisions in that particular situation.
If a BA, under a power of attorney, is the highest bidder they must immediately inform the auctioneer that they are acting on behalf of a client and provide the name of that client (section 83 of the PSBA Act). Attorneys need to also ensure that they have the original power, or a copy of it certified by an solicitor or a JP, with them at the auction in case they are asked for evidence, so that it can be sent to the Vendor’s solicitor with the copy of the contract signed by the attorney in the name of the buyer.
A BA acting under a POA can usually sign the contract on their client’s behalf as this power is usually given by the power of attorney – but the terms of the power should be checked prior to the auction.
As Alan has said already, if BA's will be taking further steps in the transaction such as executing a transfer, conveyance or a deed on behalf of the client then the power will need to be registered (stamped by Lands Office in CBD) and advice should always be sought from the purchaser’s solicitor or conveyancer for this.