I have a spare room that I want to rent out. What are the best websites for these kind of classifieds besides gumtree?
Where to post ads for room accommodation?

Comments
 - You want people to get tested if they aren't symptomatic and have no need to think they have been exposed, just to rent a share room? - Yeah, our health systems definitely going to fail faster if idiots start doing that.  - Not only that but 5 minutes after you get the all clear you can contract it. Further wasting everyone's time. 
 
 
 - On the internet.  - What's that?  - It is this thing where they connect these computers to some interweb thingy and then you send the megahurtz over the phone line and get text to your screen in return. 
 
 
 - Carsales.com.au - Seriously - Flatmates.com.au - Research .3 second google search  - Flatmates.c.a used to be the worst. All scams & scumbag agents. 
 Has it improved? - No idea , I don’t need a flatmate 
 
 - I know that there are flatmates.com.au, Craigslist, and Facebook marketplace. I just want to check if there are actually other popular avenues that I am not aware of. 
 
 - local buy, swap, sell pages on Fakebook?? 
 - Melbourne has a Facebook page called Fairy Floss Real Estate (creates by people who needed roommates). it is where a lot of my friends have found places to stay or roommates. Surely your city has something on Facebook?  - How do you find that out? I am in Brisbane.  - Friends of friends? Ask people you know who have had to find a room or roommate 
 
 

Uni bulletin boards (paper)
Uni students - especially from overseas - tend to be low-risk
(or were, B4 the Pandemic…)
The closer you are to (public transport to) a Uni campus,
the more popular your ad will be
New (since Pandemic):
Don't be shy about insisting, that the person get & show you
(B4 entering) an "all-clear" health certificate vis a vis COVID-19
I hate to add, both for Aussies & Overseas applicants:
Ask about travel (past & future desto's) &
Confirm the reply by flipping thru the person's Passport