My 6 month old sleeps in a sleeping bag, in his own cot (with nothing else in the cot). The mattress is tested and approved for the SIDS safety regulation AS/NZ 8811.1:2013. He goes to sleep on his back, but during the night he will roll onto his tummy. I roll him back over, but he will roll back to his tummy. This goes on all night. I find myself glued to the baby monitor all night to see if he has rolled to his tummy. Is it ok for him to sleep on his tummy?

Most babies will be rolling over by 4- 5 months…...some earlier, some later. 
This is normal as babies grow & become stronger.                         
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make his environment as safe as possible.
His new environment involves being in a cot (not a bassinet) & in a sleeping bag with arms out, which I note you are doing.
He needs the space of a cot & arms free to be able to roll well & safely.

**Always continue to place baby on back for sleep.
Increase tummy time when awake especially if you are concerned that he is having difficulty with rolling.

Red Nose has the following general information now that he is rolling.

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.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over