Place yeast, sugar and 125ml (½ cup) lukewarm water in a small bowl and stir to dissolve. Set aside for 10 minutes or until mixture bubbles. Place flour and ¼ tsp salt in a large bowl and make a well in the centre. Pour milk, eggs and yeast mixture into the well and stir until combined. Add currants, raisins and apple, and stir until well combined. Cover with plastic wrap and set aside in a warm, draught-free place for 1 hour or until dough doubles in size.
Fill a deep-fryer or a large saucepan one-third full with oil and heat over medium heat to 160°C (or until a cube of bread turns golden in 40 seconds). Working in batches, use 2 dessertspoons to form 6cm balls, then gently drop them into oil and deep-fry, turning halfway, for 5 minutes or until golden and cooked through. Remove with a slotted spoon and drain on paper towel.
Dust doughnuts generously with icing sugar and serve immediately.