Hi there, I was wondering if it is safe to let a baby nap during the day in their pram bassinet (Uppababy) if it is laid on a flat surface rather than using a Moses basket or a cot?

Red Nose recommends that babies are placed in their own safe sleep environment (for all sleeps), which is usually recognised as being a cot or portacot that meets Mandatory Australian Standards.
https://rednose.org.au/article/what-is-a-safe-sleeping-environment

Prams are not recommended as a regular sleep environment.
See information below provided by Red Nose & Product Safety Australia.

“Never leave your baby unattended in a pram or stroller. It is not a substitute for a cot”
https://rednose.org.au/article/prams-and-strollers

“Avoid sleeping children in prams or strollers. Never have an unharnessed child asleep in a pram or stroller as they can move about and may be at risk of falling or entrapment. If they are asleep, maintain regular supervision.”
https://www.productsafety.gov.au/products/babies-kids/kids-equipment/prams-strollers