The seller can bid at their own auction

A vendor bid is a bid made on behalf of the seller. It can lift the visible price in the room without a real buyer behind it.

Every state sets its own rules. In NSW the auctioneer can make one vendor bid, and must announce it. In South Australia it's up to three, below the reserve, each announced. In Queensland vendor bids are allowed up to the reserve, each announced.

In Victoria and the NT, vendor bids are only allowed if the auction rules or conditions of sale say so, and each one must be announced. In Tasmania the auctioneer must call each one a vendor bid. In WA they're allowed up to the reserve if the conditions of sale allow it, and nobody has to point them out. In the ACT the seller can't bid personally, and any bid for them must be declared before bidding starts.

Dummy bids, where someone pretends to be a buyer for the seller, are a different thing, and illegal. So learn your state's rules before you stand in the crowd, and listen for the announcement.