One-way car rentals are convenient. At Sydney Airport, after clearing customs, don't leave the arrival hall. The processing shop is on your left after exiting customs. After completing the paperwork, exit the hall and go to the first floor of the parking garage opposite. Cross the small road and walk straight on your right until you see a small building with the rental company's name on it. Show your paperwork receipt to the staff nearby, and they'll give you the keys.
Returning the car at Canberra Airport is even more convenient. There's a gas station nearby. After filling up, go to the blue parking lot (the rental company's parking lot). Find a parking space with the rental company's name on it, park there, and find a staff member nearby to return the keys. You can leave immediately, and your deposit will be refunded the next day.