Independent Joy Jones has written to the leader of Powys County Council, Rosemarie Harris, asking her to oppose a full lockdown for the county.
The lockdown will begin at 6pm on October 23 and last until November 9, and will see all but essential retail outlets close.
The details of the plan were contained in a letter from a regional director of the Confederation of Passenger Transport to its members in Wales, which was posted on Twitter.
Councillor Jones, who represents Newtown East, reacted with dismay to the plans. She said she feared for people's livelihoods and mental health.
"I think for Powys it will be a disaster. We're struggling as it is.
"People are struggling to keep their jobs, businesses are shutting. If they close it down it's a massive effect.
"It affects people's mental health. People's mental health, especially young people, was very badly affected last time.
'We need support, not being locked up'
"Do they really think will make a difference in Powys? We are a very spread out county, we're not all on top of each other.
"I've spoken to a lot of residents and people are really anxious about it. We really need support, not being locked up.
"It will be the worst time of the year to lock people in. A lot of people struggle with the dark nights anyway."
She said that in her experience people in Newtown have been sensible and following the rules, despite "the odd person that flouts them".
In the letter, dated October 16, John Pockett writes: "The Welsh Government will announce on Monday a 'circuit break' lockdown to begin at 1800 on Friday 23 October and continue in force until 0001 on Monday 9 November.
"We have met with officials this morning, but as this is a very fast moving situation with decisions still to be made by ministers, much of the detail has not yet been agreed by the Government."