Friday evening will see the beginning of a two week 'circuit breaker' or 'fire-break' lockdown, with the country again falling under stringent restrictions.
Russell George, Member of the Welsh Parliament for Montgomeryshire, said the action was not proportionate to the level of the virus in Powys – which has one of the lowest rates in England and Wales.
The county currently has a rate of 56.6 cases per 100,000 people – considerably lower than Shropshire, where the rate is 90, or Telford & Wrekin, where it is 122. Both are under less severe restrictions than Powys.
More Covid-19 coverage:
Mr George said families would be penalised under the Welsh Government plan, and businesses would have their fragile recovery from the previous lockdown put at risk.
Under the new rules, from 6pm on Friday, people will only be allowed to leave their homes for limited reasons – essential shopping, exercise, to provide care, go to work where necessary, or to attend schools that reopen after half term.
It means a return to the majority of lockdown regulations imposed in March, along with the cross-border confusion and complications for communities up and down Powys and Shropshire.
Mr George said Powys was being treated in the same way as some of Wales' more urban areas.
He said: "For me, it is clear that a blanket national 'circuit breaker' to combat coronavirus, such as the one that the Welsh Government will introduce on Friday evening, will be disproportionate and damaging to the people of Powys' lives and livelihoods.
"I doubt if Powys' infection rates were high, and there were lower rates in South Wales counties, the Welsh Government would have taken the same approach.
"I can't fathom how a lockdown in Powys is going to help infection rates reduce in places like Cardiff or Bridgend, so I will be formally writing to the First Minister to makes these views known loud and clear.
"The health and the economy of our nation are intertwined and while safeguarding the health of those who are particularly vulnerable to this terrible virus is an absolute priority, measures have to be proportionate to the risk."
Wales First Minister Mark Drakeford said lockdown would be "a short, sharp, shock to turn back the clock, slow down the virus and buy us more time".
He said that without action the NHS would struggle to cope.
The lockdown will continue until November 9, and will see a number of businesses, including bars, restaurants and most shops, ordered to close.
The Welsh Government also banned people from travelling into Wales from areas with high rates of coronavirus cases.
The UK Prime Minister Boris Johnson rejected a suggestion from scientists to impose a 'circuit-break' style lockdown in England last month and has instead opted for a three-tier system with differing regulations.
Schools will be impacted by the changes in Wales with secondary schools providing learning online only for the week after half-term – other than for children in years seven and eight.
Primary schools and childcare settings will remain open.
Some businesses and tourist attractions have already published notices about their closure plans with the Welshpool and Llanfair Light Railway cancelling all trips for the duration of the shutdown.
Mr George added: "This national lockdown will unnecessarily penalise the people of Powys from seeing their families and loved ones which would in turn have a detrimental effect on people's health.
"It will also prevent businesses, especially in the hospitality sector, from recovering from a disappointing summer season so it is vital that the Welsh Government’s decisions to implement so-called 'circuit breakers' are proportionate to the threat and minimise damage to any hopes of economic recovery.
"An urban solution which may be suitable for South and North Wales isn't right for rural Mid Wales.
"Where we have had cases in Powys, these have been dealt with quickly by Powys Teaching Health Board and this is how we are going to beat this virus – smart, hyper-local lockdowns to reduce the spread of Covid-19 whilst minimising the economic and social impact of restrictions."