Dear Crownofgold,
All agents tend to encourage potential buyers to make offers to buy before auctions as this is the best way to evidence the level of market interest on the individual property offered for sale.
If the offer made falls within the selling range, it may be either used to adjust the reserve price upwards or, if it falls short of the reserve price, discarded altogether.
Generally a vendor would only sell before auction if there is little market interest on the property and they're certain that they can't achieve a higher price through a competitive auction environment.
Once you have made an offer to buy before auction, it will not be confidential.  The agents are free to communicate your offer to other buyers  in order to encourage them to offer more.
To counter this possible scenario, have just that little bit more $$$ in your kitty than what you would be offering. You might find it very handy come the auction day in case your offer is rejected,  Or if the vendor comes back with a counter-offer.
PS. Couple of other things:
1. a vendor has a right but not an obligation to sell, meaning they can reject your offer and are not obliged to give a reason why
2.  if it gets to auction scenario, remember to slow the auction down at every opportunity (ie if auctioneer is asking for $10K advances and you are one of the main bidders, keep offering them $5K's etc.)
Wishing you best of luck and nerves of steel.
m