It's not just an Australian thing.
It's a regional/generational thing here in the U.S. I was born in 1954, and grew up calling my parents' and grandparents' siblings and spouses as Aunt Helen, Aunt Anita, Aunt Anna, Uncle Walter, Uncle Fred, etc. My kids and my siblings's kids were raised to refer to my siblings and me and our spouses as Aunt Donna, Uncle Jim, Aunt Tammy, Uncle Kevin, Aunt Laura, and Uncle Bob.
My grandchildren refer to their aunts and uncles just by their first names. When I talk about my kids to the grandchildren, I use the Aunt or Uncle title in front of the first name. OldestDD's DH's name is David Latimer. My son's first name is David. I usually refer to them as either Uncle David Webster or Uncle David Latimer.
=>To add even more of a twist to my family's story............
OldestDD met her now-DH during the preparation time of MiddleDD's wedding to David Latimer's first cousin Jonathan. Sisters that marry first cousins and have children result in a happy close family.
The children are nieces and nephews to the men because of the moms being sisters AND the children are 2nd cousins to the men because the 2 men are 1st cousins.