The buy/sell will be matched at the common bid on open.During...

The buy/sell will be matched at the common bid on open.

During pre open, buyers/sellers can put in bids but there won't be any trading.
That's why (assuming you use commsec) you'll see an 'indicative price' with a surplus volume.

When trading opens, the matching bids are settled and the day kicks off.

Putting an order in above the highest current bid simply means you skip the queue and you buy that amount of shares available at market price based on the sell queue at open.

Example.

Close at 1c.

Sell orders for 100 exist at 1.1c, 100 at 1.2c and 100 at 1,3c.

Buy orders exists for 600 shares at 1c.

You put an order in to buy 200 shares for 1.5c on open, you'll purchase the 100 shares at 1.1 and 100 share at 1.2 and the lowest available sell order on open will be 100 shares for 1.3c.