Hi, can I put my newborn on his mattress on the floor for day sleeping? If so, what age is this suitable to?

Red Nose recommends the safest place for baby to sleep, for all sleeps, is in a cot that meets Mandatory Australian Standards on a firm, clean, well-fitting & flat mattress.

Is there a specific reason you want to place baby for sleep on a floor mattress for daytime sleeps?

There are many potential risks that need to be considered when baby is placed on the floor for sleep.
Some babies will wriggle & move around from a very young age, which may find them exposed to risks, such as the following.
Exposure to blinds/curtains/power outlets.
Is there risk of entrapment between mattress & wall?
Are there stairs nearby?
Are there other young children?
Or pets?
https://rednose.org.au/article/making-your-home-safe-before-moving-a-child-from-a-cot-to-a-bed

Safety organizations recommend that babies remain in a cot for sleep, until they attempt to climb out. This is usually around 2 -3 years or may be as early as 18 months.

Red Nose does recommend room sharing for all sleeps, but not at the expense of a safe environment for baby to sleep in.
Red Nose has this helpful information.
Parents are not expected to observe their baby constantly. If your baby is sleeping in a separate room check baby regularly to ensure that the baby remains on the back and the head and face remain uncovered (as baby grows beyond 5-6 months they will move around the cot and roll over; settle baby to sleep on their back but let them find the sleep position they feel most comfortable in. A safe cot and safe sleep environment is still necessary for babies older than 5-6 months).
https://rednose.org.au/article/room-sharing-with-baby

A porta cot would be the preferable environment for baby’s daytime sleeps downstairs.
Ensure it meets Australian Standards. (AS/NZS:2195)
https://rednose.org.au/article/portable-cots