They send you a reminder just before the year is up, but (helpfully) the letter doesn't actually tell you what your deadline is.
I rang the helpline, and they couldn't tell me the date, but arranged to send out the letter that your health fund needs to confirm no loading is payable.
I knew I was close, so instead of waiting I went into my local office. After a lot of searching they managed to find the date and produce the letter. (The date is apparently on the 'Group' screen on the system, under 'Date association began' or something similar.)