Hi, my almost 6 mo is determined to sleep on her tummy during the day and night. We always put her down on her back but she will move to be either on her side or tummy. I’ve woken up in the night to find her completely face first in the mattress (which is firm). Am following all other guidelines here re bottom sheet, no bumpers, safe sleeping bag etc. Her nose is 100% squashed in to the mattress and she won’t turn her head to the side (even though she can). I feel this isn’t safe sleeping but what can I do? Should I be worried?

What you are describing is normal development.
Most babies will be rolling over by 5 months…....some earlier, some later.
This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy, & often don’t change from that position.

This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur.

Always place baby on their back to sleep.
Ensure cot mattress is firm, well-fitting & flat…....... this is especially important when babies occasionally “face plant”
Ensure there is nothing loose or soft in the cot.
Use a sleeping bag now that has appropriate size neck & arm holes.

Red Nose has the following information now that your baby is starting to roll.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position