FCL.940.FI - FI Revalidation and Renewal
(a) For revalidation of an FI certificate, the holder shall fulfil 2 of the following 3 requirements:
(1) complete:
(i) ... if the privileges to instruct for the IR are to be revalidated, 10 of these hours shall be flight instruction for an IR and shall have been completed within the last 12 months preceding the expiry date of the FI certificate;
If you haven't got the hours, then the options are as for any other FI rating - (2) seminar and (3) flight test.
What training you might need to successfully complete the flight test would be up to the ATO and your examiner.