Winters vary quite a lot across Australia.
Budgies in the wild would have a temperature range of -5 to -15C rising to 30 to 40C
Many aviaries are outside with temperature of -5C to 15C in winter.
Some owners do bring birds inside.
If you think your birds are suffering from cold try bringing the inside and cover their cage with a towel when it gets dark, taking it off when you get up in the morning.
2 reasons why this is done 1 is to keep the birds warm and the other is to keep them quiet at night