Wallpaper paste has anti fungal agent so it prevents moulding.

Wheat paste is also a good adhesive but does not have antifungal agent. So if you are in a humid climate this would not work as well. Unless you don't need to keep the paper mache product.

I would like to ask though: Is it possible to make wheat paste anti fungal by mixing it with wall paper paste?

Some websites suggest adding salt but does that work?

I am in a humid climate and on a tight budget so it would be helpful if anyone could let me know.

PVC wood glue works instead of ?size? & is less messing about! Even mixing some in poster or plastic paint,  will help to bond, strengthen &/ or seal  the surface.
Salt does work for wheat or flour glue, if you have not got any PVC or size glue on hand.

Your right Nick. Alum has been used for centauries in hide Tanning  & dyeing cloth, because of it?s antifungal & otjer Properties. The ancient Egyptians & Turks used this in many processes, rather Than ordinary salt. In fact it was highly valued by the 'Pohms' in their cloth industry in Yorkshire, for fixing & setting when dyeing cloth, before it was hung on tenterhooks outside to dry. {I was going to say ?in the sun? but in Britain this would have to be an oxymoron!} [Because it was so highly prized in the cloth industry, I believe the Turks used their vast Aluminium Phosphate deposits & refined it with urine. So they could flog more of it in the market place - So I?m not sure if this process would have been anti bacterial as well?].