EGM Question List, page-34

From the comsec site

How do I manually exercise an Equity Options?
To exercise a Equity Options contract, you need to notify CommSec prior to 4.20pm Sydney time for it to be exercised on that day. We will only accept verbal instructions to exercise the contract. CommSec will then notify ASX Clear who will randomly determine which writer will be allocated to the trade.

You can contact an Options Representative on 1800 245 698 or +61 2 9115 1532 if calling from outside Australia (Monday to Friday, 8:30am to 5:30pm Sydney time).