Sarah would be paid time and a half for working on a public holiday, but she would not also get an alternative holiday, because Saturday is not her normal day of work.
David also works at the café. He normally works Saturdays and Sundays.
If he works on the Saturday public holiday, he would be paid time and a half, and he’d also get an alternative holiday, because Saturday is his normal day of work.
If David was on annual holiday and didn’t work on Saturday, he’d still be paid for the public holiday.