You carry forward your original cost base for the acquisition of...

You carry forward your original cost base for the acquisition of Newcrest and given that you have held shares for more than 12 months you get 50% capital gains tax discount ie say your original cost of Newcrest was $10,000 and you sold Newmont for $15,000, you have a gross capital gain of $5,000. Apply the 50% discount and you pay tax on $2,500 at your marginal tax rate. If you have any tax losses you must apply that first before applying the discount

Be sure you get your own tax advice first, the above is my knowledge of tax law from experience. You can readily get this information by using a tax calculator from the ATO website