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[-Peter]

Chelodina longicollis live in bodies of water that get down to single digit temps over winter. They cease to eat at this time and will aestivate over winter either on land or in the water. They can still be active at 10 to 15 degrees C.
Captive juvenile turtles on the other hand have the disadvantage of not being able to locate the best possible site to go into torpor so must rely on you the owner over the cooler months.
Thus I would suggest you get a heater and move the tank away from the window if it is in direct sunlight.
Hornet is correct in stating that a lot of turtles die from overheating due to being in direct sunlight.
Check that the thermostat on the heater is correct before using it. I had one that heated the tank to 50C on the factory preset.