Yes. You can return a car hire early, but whether you can get any refund, or even get charged for the early return, will depend on the terms and conditions laid out in your rental agreement.
By returning the car earlier than you specified on your rental agreement, you can be charged for the amount originally agreed upon at the time of rental. A few companies will recalculate your rental rate (while others do not) based on the number of days rented and charge you the recalculated rates — which may be higher or lower than your original rental quote, depending on any discounts you may have been quoted.
Some suppliers may charge you an early return fee, and may or may not refund you for unused days. For example, if you booked a rental car at a special rate because it was for a 7 day period, but now you want to return the vehicle after four days - you would be charged the higher daily rate for those four days instead of the discounted weekly rate. The reason for the charge is "to compensate the rental company for its inability to rent the car during the time reserved for your use," as Hertz puts it. According to Hertz Australia website, early returns incur a $30 fee (excl taxes) if returning earlier than booked by 24 hours.
Read more about returning your rental car.