Tasmanian Greenpeace activist Colin Russell, who is being held in jail in Russia, says there would be outrage if prisoners were kept in similar conditions in Australia.
Mr Russell, 59, is one of 28 international activists and two journalists who have been detained by Russian investigators for piracy and hooliganism charges, which carry sentences of up to 15 years.
The group was detained in the arctic city of Murmansk but may be moved to St Petersburg.
Among those facing charges are two freelance journalists Alexandra Harris from Sydney and Jon Beauchamp from Adelaide.
Russia seized the Greenpeace ship Arctic Sunrise and detained its multinational crew in September, after activists attempted to storm an oil rig in the remote Pechora Sea.
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In a letter to his wife Christine and daughter Madeleine, Mr Russell, who is kept in his cell for 23 hours a day, describes the below freezing conditions in Murmansk.
He says the iron bars on his bed cut into his back.
"They don't give you anything at all. I am ok with it but there would be outrage if this was at home," Mr Russell said in the letter.
"I do not want to think of what happens if you are convicted. God knows what they do with the real crims."
Mr Russell has lost weight due to the poor food.
Greenpeace says that oil drilling in the Arctic is dangerous and risks destroying the sensitive environment.
Greenpeace spokesman Reece Turner has said the conditions in the Russian prison are inadequate.
"We urge Foreign Affairs Minister Julie Bishop to do all that she can to make sure there isn't an empty seat at the Russell family table this Christmas," Mr Turner said.
