It seems sushi has gained citizenship in Australia. You can find sushi shops everywhere and many people eat sushi for lunch. Even some school canteens sell sushi for lunch. What are your favourite sushi fillings?
My family will say: avocado, cooked tuna, teriyaki chicken, teriyaki salmon, teriyaki beef, fried prawns, etc.
Unfortunately, it is very unlikely that you will encounter any of the above in a specialty sushi restaurant in Japan although you may find something like that in convenience stores and/or sushi train type of restaurants where many Western tourists eat in.
In Japan most sushi rolls are much thinner than those in Australia and usually has one filling only. Common fillings are: raw tuna, eggs (sweetened omelette), cucumber, takuan (pickled daikon radish), nattou (fermented soy beans), ume_shiso (pickled plum paste with Japanese basil leaves), etc.
If you see fat rolls, they are usually fatter than those you see in Australia and are served sliced like a disc. They are usually filled with multiple items unfamiliar to Westerners such as kouya_douhu (reconstituted freeze-dried tofu) and kanpyou (reconstituted dried gourd shavings). This type of fat rolls is usually eaten without dipping into soy sauce.
As for nigiri, in Australia, salmon and tuna are the most common toppings, but in Japan salmon is not as common as raw tuna. Among raw toppings bonito, king fish, snapper and/or squid are probably much more common than salmon. In Japan fish cured with vinegar and/or cooked with a sauce are often used as a nigiri topping too.
One more thing I want to mention is that “sushi rice” needs to be vinegared. If rice is not mixed with vinegar, no matter how much it resembles sushi, it is not called sushi. Rice balls without vinegar is called “onigiri （おにぎり）” and it is a more common for lunch in Japan than sushi.