I'm currently looking at booking flights to Singapore around the 9-15th of January and have found that the prices at this time are much higher than usual - about 3-4x higher than the upcoming March deals (https://www.ozbargain.com.au/node/740480).
This brings me to my question: with respect to price, is it typically better to buy flights as far in advance as possible or wait until as late as reasonably possible (with the caveat that you still book while there is availability remaining)?
You are also looking at school holiday times, which are notorious for sky-high pricing.
Leave it until the day before so we get to see the panic post about how unfair it is you didn't get tickets.
If you are desperate to go, you'll have to suck up the extra cost.
Nah, book one way only and then wait till last minute to buy the return leg and then post in panic saying how prices are sky high and you can't afford to flyhome.
As far in advance as possible. Occasionally it may become cheaper an airline has a sale, but it will highly depend on when you plan to travel, and your desination (and the airline of course). You also should consider when the peak travelling periods are either domestically (due to school holidays etc) or based on the destination (good season or rainy season, etc)
For popular routes waiting until the last minute will almost always be the most expensive time, if there are any seats left available.
This is during school holidays, so fares are much higher. My wife and I are going to Vietnam mid-late January (leaving on Jan 18) and got very good fares with Singapore Airlines. You're probably just a bit too early for the lower fares.
As mentioned, school holidays and Chinese new year is the worst time to fly.
But also an airline wants as close to full flight as possible and will drop flights/reorganise if they can't get it. The way to achieve this is to get as many seats filled as early as possible, to give an indication of what the flight will be like, so they will be cheaper early on to encourage booking. This has the benefit of a higher likelihood of cancellation too, which means more revenue.
As consumers, we want to book as late as possible, so they can charge a premium for that.
The best way to do it is decide early when you want to fly then look for a price drop below average for the period leading up to that. Don't look for a price lower than the lowest price leading up to your booking, because you probably won't get it, thus aim for below the average instead. So if it has averaged $1,000 leading up to your booking, the minimum was $800, don't wait for $800 again, book at $900 if you see it. You might not get the absolute lowest price but you have the lowest risk of paying more.
Budget airlines sometimes drop their prices closer to departure to fill seats but there is no way for us to know how empty a flight is (looking at a seat map doesn't tell you that) and if it's likely to be discounted so it's all a gamble really. Book whenever you are comfortable with the price, taking into account extra demand during special events and holidays.
Google Flights https://www.google.com/travel/flights is a tremendously useful facility for planning your travel.
We used tracking for a particular route (Perth to London Heathrow), and were getting price graphs from them each day showing the cheapest prices for the next 3 months. This seemed to show that booking just before the travel date would be expensive, but booking a few months before traveling would be cheaper (half the price!). May be a different story for other routes.
There is a useful tutorial here https://flighthacks.com.au/how-to-use-google-flights/
This might be due to Chinese/Lunar New Year, which is on the 22nd of Jan.