Mark Drakeford said he was forced to impose new rules in Rhondda Cynon Taf - which include social restrictions and business curfews - because people had "forgotten just how serious all this is" after the national lockdown was lifted.
Speaking to the Acting Prime Minister podcast, the first minister said people had taken the lifting of restrictions as a "sign somehow coronavirus has disappeared".
For example, he said, in Rhondda Cynon Taf "whole groups of people have got together and not behaved in a way that I think you could describe as responsible".
Drakeford on the reasons behind Rhondda's lockdown:
That's resulted in "large numbers of people contracting the virus, and then that virus being spread further", he said.
He attributed the rise in Covid-19 cases in Wales more generally to the return of people from holiday in virus hotspots, and to household transmission, which led to him implementing the new 'Rule of Six'.
He told podcast host Paul Brand that following lockdown some people had "dropped their guard" and went back to visiting one another in each other's homes.
The first minister provided a nugget of hope for people in Rhondda Cynon Taf, with a suggestion that the local lockdown in Caerphilly was beginning to work.
He said: "There have been some very tentative signs, but they are on the positive side, that we may have reached a plateau of infections in Caerphilly."
Are lockdown restrictions working in Caerphilly?
Cases there had been "going up exponentially" but lockdown restrictions may have "slowed that rise and begin to see it flatten off," he said.
He warned that figures can vary day to day but said over seven days they show that "probably things are slightly better than they were".
Mr Drakeford said he would like to avoid a return to a national lockdown in Wales, because in half of the country "the virus is still going down".
He said, with Covid-19 declining in 13 of 22 local authorities, a "national lockdown at this moment would make no sense at all".
But he suggested a larger lockdown could be enforced in the south east corner of Wales where there are some "difficult spots".
Drakeford on the chances of a Wales-wide lockdown:
He said, because of the way people travel between areas, he's having "to watch the whole of the region in that way".
"If we had to act regionally, because there's evidence that this is now going beyond individual local authorities then we would have to face that," he added.
Mr Drakeford revealed dealing with the government in Westminster during the coronavirus pandemic has been a "frustrating" experience.
He said he's "tried to argue all the time for more engagements" but its been difficult to arrange meetings with representatives from all four governments.
Dealing with Westminster has been 'frustrating'...
Instead of regular meetings, he said there's been a "pattern in which the UK government comes to the table and then disappears from the table".
He added: "We've not met this week, which I find baffling. Here's a week in which we're all having to make really difficult decisions, we're all facing coronavirus rising again in different ways and in different places.
"Despite the fact that I know the Scottish government asked on Monday for a meeting, and we would have been part of that meeting, that meeting just hasn't happened."
He acknowledged ministers are busy but said he feels there are "parts of the current UK government who have discovered devolution during the pandemic and don't like it".
Some parts of the government don't like devolution:
"Deep down, and sometimes not that deep down, they think it was all a terrible mistake," he said.
He added how they "want to get back to a position in which make the decisions, they don't need to talk to anybody else".
"Unfortunately that is flying in the face of the realities but I think it's quite a strong tendency is some parts of the Conservative Party," he said.