I’m fairly sure the Singapore Airlines site allows you to check the extra charges associated with KrisFlyer bookings without having the prerequisite number of points.
However with Qantas… If you try to book a paid fare with them and towards the end of the precess look for an option to display the price breakdown. This displays the taxes that you’ll have to pay as a minimum. Plus, I suspect, you’ll have to the carrier imposed charges too.
The other option is to look at Matrix which will give you full fare breakdown. Look for the taxes and other fees portion of the ticket when you get to the results screen. You can’t book anything through matrix but it’s a useful fare research tool.
Or give Qantas a call, they’ll help you pretty quickly.
I normally open a new browser window in private mode and look at the fare breakdown for a paid fare. The taxes and surcharges carry over if you use points for the base fare.