There is no special field in Tax Return to opt out of the averaging system. 

Please fill Other Attachments Schedule with relevant information. All text from this field will be sent to ATO. However, to be sure I would recommend the user to contact ATO and ask what to do in this case.


Below is the ATO related link and how to contact them directly thru letter: 
https://www.ato.gov.au/businesses-and-organisations/income-deductions-and-concessions/primary-producers/in-detail/tax-averaging-for-primary-producers