In Australia we don't get the same extent of snow as many other places, and therefore no curtailed riding season. We can ride all year round, even middle of winter.
However, it does get plenty cold enough here, and most experienced riders go with the following thermal underwear for year round riding.
For those who prefer natural fibres - Woolarina
Finest Australian Merino wool.
Welcome to Woolerina - 100% Australian Merino Wool | Lightweight Wool Clothing | Durable, Breathable Wool Clothing for Men, Women and Children
For those who prefer synthetic -
Damart - Fashion and thermal wear
I've got products from both of the above, and both get my high recommendation.
The synthetic is slightly warmer for the same amount of bulk.
The wool is slightly more comfortable.
The synthetic wicks more moisture away from the skin, and washes and dries faster.
I experience a few static shocks when wearing the synthetic garments. Take them off and my body hairs stand on end.
I've ridden 400 klms in perfect comfort at -3 to -10 degrees celsius, with the following clothing items -
Feet - Damart long socks under a pair of DriRider boots
Bottom half - Damart Grade 5 Double Force Longjohns, a pair of kevlar jeans, and a pair of waterproof pants to keep out the wind chill.
Top half - Woolarina longsleeve thermals, longsleeve/high neck cotton riding shirt, Sleeveless polar fleece vest, Rivet Monaro riding coat
Head - Damart balaclava
Neck - RJays Polar fleece neck warmer
Hands - BMW silk undergloves + DriRider Hurricane gloves
Heated handgrips.