There have been a further 165 cases of Covid-19 in Wales, bringing the total number of confirmed cases in the country to 18,829.
Public Health Wales said no further deaths had been reported, with the total number of deaths since the beginning of the pandemic remaining at 1,597.
Wednesday's daily figures showed there were 33 new cases in Caerphilly, followed by 20 in Rhondda Cynon Taf, and 18 in Newport.
Rhondda Cynon Taff has seen 100 cases in the last seven days.
Its local MP Chris Bryant warned the area is in "real danger" of following Caerphilly if the situation does not change in the coming days.
There was one case of coronavirus confirmed in a hospital setting, PHW figures show.
Dr Giri Shankar, Incident Director for the Novel Coronavirus (COVID-19) outbreak response at Public Health Wales, said they are "concerned" by the rise in positive cases in Merthyr Tydfil and RCT.
“Our investigations indicate that a lack of social distancing by people of all age groups in a range of different locations has resulted in the spread of the virus to other parts of the population.
"We continue to appeal to everyone to please remember that, even if you feel that you would not be badly affected by COVID-19 if you were to test positive, should you pass it on to older or more vulnerable family members, friends or colleagues it could be extremely serious and even fatal.
“Coronavirus has not gone away. It remains the responsibility of everyone to help prevent the spread of this virus – that is, by self-isolating when asked to do so, staying two metres away from others, and by washing hands regularly."
After Dr Shankar said he was concerned by the rise in cases in Merthyr, the leader of the local council Kevin O'Neill has stressed the importance of people following government guidance to help 'protect' the community.
"With the recent announcement of the local lockdown in Caerphilly, I cannot stress enough the importance of residents adhering to Welsh Government and Public Health guidance to stop the spread of this virus across our communities," Cllr O'Neill said.
"It is vital that we remember how quickly this virus can spread and we need everyone to play a part in preventing it from spreading.
'Clusters of positive cases have been reported in Wales stemming from people returning from holidays who did not self-isolate for 14 days as required.
"The reckless behaviour of a few can put some of the most vulnerable people at risk of infection.
"Please help us to keep our communities safe."