As you are in Melbourne I'm not sure, up here I only need to heat part of the dry end of my knobby tails tank and he hides away in his sand tunnels all day in the moist half and doesn't come out onto the heated section until late at night, so I can afford to turn it off during the day if I wanted to (I don't though).
I got the impression of various threads and posts from more experienced members that a lot of reptiles needed some heat (the temps vary with sp and there are some sp people agree on no heating like eastern blue tongues, mixed opinions on sp of gecko's though).
I've used a cheap chinese thermostat hooked into a ceramic heat bulb for months at a time at most so far without any issues, you can usually adjust the temp in them if they are a out a degree or 2.
Going off about 6 different care sheets for knobby tailed geckos I found they all say about needing a heat gradient, some said 26 on the cool end to 30-32 on the heated end, others said about the temp gradient but only mentioned the heated end temp to keep with no temp limits mentioned for the other side of the tank.
Only 3 of the 6 mentioned about gradually lowering the temp of the heated end for winter.
1 said down to 17 at night, 24 during the day for winter.
Another said a similar thing but down to 20-23 degrees in winter.
Another said they don't tolerate temps above 30 for long and recommended a temp gradient of 22-27 degrees (most say around 30 degrees).
All of them mentioned using a low temp heat source like a heat pad/cord under the dry end or a blue/red bulb above a hide/dry end (I'd go the blue if you do go either bulb, hadn't seen many mention the red ones, a few people here use other types they'd be able give you advice on).
1 made a the very good point of "Most parts of Australia stay within this temperature range for many months of the year, and so heating may not always be needed – but it’s certainly recommended to have a heat source for your gecko if you’re in the United Kingdom" (the uk being the example as this was off a .com site not a aussie site).
With how variable the temps can be in Melbourne and how extreme they can go I would stick with a heat source in part of the dry end/above a hide hooked into a thermostat with a set lowest temp and highest temp for it to reach, ie 24 lowest temp in winter, highest 33, 27 lowest the rest of the year, 32 highest. I would keep it running all the time in winter, only times off in summer when in heatwaves and if bud doesn't go to the heated end until night time regularly I'd turn it off during the day but back on if it got cool.
1 said they can tolerate temps even down to single figures in winter, being a desert sp I would believe this but still be careful not to let the temps get that low just in case.
So going off the facts that did seem consistently mentioned I would stick with atleast 24 degrees up to 32 degrees for the heated end, only gradually lowered down as low as 24 in winter, 32 the rest of the year.
I believe they would be able to handle down to 20 because of the temps the sand would get down to at early in the night in winter but won't recommend it as I'm new to them too and only 2 said as low as 17-20 for the cooler temps for the heated end in winter (going off several posts on the topic and other caresheets, not just these 6).
People with a lot more experience than me will be able to give advice on the temps they do down to for winter/if they do and the temp they find the gecko's like for the heated part, my advice is only based off what facts seemed to be the most consistent from multiple caresheets, which can be written by experienced keepers/breeders but some could be facts gathered from multiple caresheets they checked out with little experience like I just did.
So I'd go with the facts that seem to be consistent across multiple sheets and threads and posts from experienced people, that's what I did when fact gathering before I got my gecko and blue tongues (hadn't been as thorough with my woma though and a little slack with replacing her smaller heatpad with a larger one and learnt more about the effects of inadequate heating recently but with pets you are always learning).