I found this on Reddit

Reddit user SandGlen:

 

I've just made a tool that you can use to check the number of unsold fares (aka loads) on any flight.

 

If you're bidding for an upgrade, it helps to check the remaining availability by fare class before submitting a bid. It works with most carriers, not just Qantas.

 

It's free, but requires you to sign up for an account. The data is pulled live from the GDS, which costs me money.

 

Flight load checker: https://flightseats.io/flight-load-checker

 

A free account gives you 5 load searches per month, paid subscriptions for more are available.

It does of course require you to give away your email address (seems fair enough as it's not free for the dev to pull the info).

It's Qantas centric but I tried it with an AirNZ flight I'm taking next month and it seems to work.