The ATO has advised as follows:
"Your client's tax return may be held for review before a notice of assessment issues. Where a return is held for review, we will write to you seeking further information to substantiate the claims contained with the return.
To enable the review to be completed as soon as possible, you need to provide a response by the due date in the letter.
We will endeavour to complete the review within four weeks of the due date for response. Therefore you must allow this amount of time before enquiring about the progress of the review.
For more information, refer to Income tax return integrity supporting documentation."