If you are trading gold for silver or vice versa that is a taxable event, no different if you sold BHP shares and bought some Evolution shares in exchange.
As to swapping one form/size for another but of the same metal, I'm not sure of the treatment. I know some investors have had advice that would be taxable but the ATO may consider a 10oz bar and 10 x 1oz bars to be two different products so a taxable event. If you got the exact same ounces back and didnt have to pay any difference due to premiums then you may be able to argue that the swapped products were equivalent.
Note that one advantage if a gold for gold swap is taxable is that it allows you trigger a taxable event when you have gains or losses you want to realise to offset against other losses or gains without losing any price exposure to to gold.