Hey there, I just read this resource (https://rednose.org.au/article/why-back-to-sleep-is-the-safest-position-for-your-baby) but I can’t see anything about babies sleeping on their tummy in your arms or on your legs I.e. supervised. My 5 week old slept for 1.5 hours this morning in my lap, on his stomach. Is that safe or should I move him from that position immediately once asleep? As in, is it unsafe to leave them on their tummy unsupervised (wouldn’t do that) or is it unsafe period?

Placing baby for sleep on their tummy is unsafe. Young babies, when sleeping protect their airway best when sleeping on their back.
It is ok for you to nurse baby, but once he is asleep, place him on their back in his normal safe environment.
Often parents have rested on a lounge with baby on their tummy & the parent has gone to sleep as well. This creates further risk.

Red Nose has this information:
There is a very high risk of a sleeping accident if an adult falls asleep with an infant on a sofa. This is because baby may become wedged into cushions or the back of the sofa and the sleeping person would not notice.

Never fall asleep with baby on your chest whilst lying down as this is the same as sleeping the baby in the tummy position. Sleeping in this position is dangerous as baby’s airway is not protected in this position, which could lead to accidental suffocation. Baby should also be placed to sleep on their back, on a firm flat mattress with their face and head uncovered.

Place baby on their back in their own safe sleeping place before you doze off on a sofa.
https://rednose.org.au/article/does-sleeping-with-baby-on-a-sofa-or-couch-increase-the-risk-of-sudi