The first bottle is being auctioned off on eBay and bidding is up to $18,000 ($23,000 Australian dollars).
Additional bottles containing just 25 milliliters of the secretive sauce will go on sale for 50 cents a piece at 950 McDonald's locations throughout Australia this month.
McDonald's hasn't revealed whether the sauce would be sold in other countries.
The burger chain has started running a number of promotions in an effort to boost lagging sales.
In the US, the company started accepting a new form of payment Monday that could include selfies and hugs.
The promotion is meant to highlight the chain calls "lovin'" ad campaign. Customers will be chosen at random to participate in the deal.