I want to buy a sleeping bag for my 2.5 month old ready for Melbourne's winter and her being able to roll. I am looking at sleeping bags which also have long sleeves. 3.5 TOG. Would this overheat her?

Red Nose does not recommend any specific amount of bedding for baby.  All babies are different, & different homes & areas will have temperature variance..
TOG ratings can be a guide, but even that might vary for each baby.
It is important not to overdress baby & to always keep baby’s head & face uncovered, which is important to prevent overheating.
Red Nose has the following information.
Overheating can be caused by room heating, high body temperature, excessive clothing or bedding and head covering.
To reduce the risk of this, Red Nose recommends that you, as baby’s parent or carer, use your own judgement, taking into account factors such as where you live (climate, whether it is summer or winter), whether you have heating in the house, and whether baby has a cold or minor illness (which may cause their temperature to rise).
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.
We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.

To check, feel baby’s tummy, which should feel warm. Don’t worry if baby’s hands and feet feel cool – this is normal.
https://rednose.org.au/article/how-to-dress-baby-for-sleep