IO, he tells us he already has a UK licence that is lapsed, so need for the conversion.
Cessna boy. It is very simple. Find an examiner and do a revalidation skills test. Just the same as a UK pilot who fails to get the revalidation signed off in time.
If you are current on your Aus licence there is no problem and I would be surprised if you need any training and as IO points out in the meantime you can fly on your Aussie licence.