From the 28 Degrees website
How is the foreign exchange rate calculated?
The foreign exchange rate is calculated by MasterCard International on a daily basis according to daily foreign exchange fluctuations. The conversion will consist of two calculations if the transaction is made in foreign currency – it will first be converted to US dollars and then calculated to Australian dollars for listing on your transactions.
From the Visa Website
How does Visa calculate its rate?
Every day, except weekends, Christmas Day and New Year's Day, Visa calculates the rate for the next day's transactions. The Visa rate is selected from a range of rates available in wholesale currency markets or the government-mandated rate in effect one day prior to the applicable central processing date. Visa makes this rate available to issuing banks, which may adjust the rate in billing cardholders. The rate Visa makes available to issuing banks may vary from the rate Visa itself receives.