Hi Jim,
The best way to calculate what you'll spend on food is to have a look at the menus on the dining pages of
www.allears.net It lists every Walt Disney World restaurant, table service and quick service, and the prices. I'm not sure what the portion sizes are in Australia but here in England they're much smaller than US portions so as a family we will often share meals at quick service places. Sometimes 2 meals is enough to feed the 4 of us.
As a rough guide a quick service meal will cost roughly $7 for a burger and fries, $3 for a dessert and $2 for a drink. The table service restaurants vary quite a bit but a dinner buffet at Crystal Palace in the Magic Kingdom is approximately $30 for adults. A non-buffet restaurant like Kona Cafe at the Polynesian will cost roughly $20 for an entree and $6 for dessert per adult for dinner.
Another great way to save money is to buy a refillable mug at your resort. For the whole of your trip you can get free refills of normally soda, tea, coffee and still lemonade at the resort it was purchased in. At the moment they cost $12.49 plus tax and as you're staying for 10 days it is great value.
I hope you have a great trip,
Jo