ie; if he keeps it to long and is subject to capital gains tax when he sells he will lose a decent portion of any gain . If he has only recently rented it out then he is ok and would gain a nice profit tax free on sale which he could then use as a nice deposit on a suitable house .
Without knowing his present income and end objective as stated above it really is best to seek an acct's view .
To answer your other question re 1992 I could never see a unit or any unit going back to 1992 prices in Australia . Labour costs are 2 or 3 fold since that time frame . The cost of material to build now is substantially greater as well . By the sounds of recent sales it is doing rather nicely and as a young fella he should be congratulated .
cheers ps; I don't know Brisbane although have a brother up that way and I just did a search on Toowong and it looks lovely near the botanical gardens , major hospital close by , river close and it only looks 4-5 k's from Brissy cbd plus recent sales on realestate.com show many unit sales well above what u said it's worth . Looks like a prime spot hence no major property crash in the area and DEFFINITELY will not go back to 1992 prices [PHYSICALLY NOT POSSIBLE]
