Our baby has significant reflux issues whenever sleeping on his back (he sleeps fine when sleeping supervised on his side or at an incline such as in a car capsule). We are aware of the safe sleeping rules for normal mattresses but wondering if there is any official position on whether mesh mattresses like the numu (https://www.numubaby.com.au/products/cot-mattress) allow for safe sleeping on the side.

How old is your baby?
Has he been formerly diagnosed with reflux?
Have you discussed this with your GP, Paediatrician or Health Nurse/Midwife?

Red Nose does not recommend or provide information for specific brand name products, like the mattress you are referring to.

Remember that the recommendation is to always place baby on their back for sleep on a firm, well-fitting & flat (not elevated) mattress.
This is because we know that this helps to protect baby’s airways.
https://rednose.org.au/article/what-do-the-safe-sleep-recommendations-mean
https://rednose.org.au/article/sleep-baby-on-the-back-from-birth-not-on-the-tummy-or-side
This article has a comprehensive explanation of the importance of being placed on back for sleep.
Download Why Back to Sleep is the Safest Position for Your Baby

Here is some specific information in relation to reflux.
https://rednose.org.au/article/my-baby-has-reflux