Mark Drakeford used WhatsApp to "seek clarification of the rules" during the pandemic, the COVID inquiry has been told.
A legal representative told the inquiry that WhatsApp disclosures showed Wales's outgoing first minister was "regularly using [the messaging platform] to discuss policy announcements and even to seek clarification on the rules".
Nia Gowman, who represents Covid-19 Bereaved Families for Justice Cymru, said WhatsApp messages disclosed to the inquiry showed then health minister, Vaughan Gething, and first minister's most senior special adviser "turning on disappearing messages".
Read more: The families in Wales who want answers on COVID decisions
"The limited messages that have been disclosed clearly show WhatsApp and text messages used to discuss government business where they shouldn't have been," Ms Gowman said.
"They show Welsh government senior special advisers suspiciously and systematically deleting communications.
"They show special advisers reminding themselves, and others, that they had agreed 'to clear out WhatsApp chats once a week'."
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The UK COVID inquiry held its first hearing in Wales on Tuesday, looking at the decisions taken by the Welsh government during the pandemic.
The inquiry will sit at the Mercure Hotel in Cardiff until 14 March.
It was confirmed at the hearing that both Mr Drakeford and Mr Gething will be called to give evidence, as well as senior government officials.
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Some people who lost family members during the pandemic have told Sky News they want to see a separate Wales-specific COVID inquiry, as is the case in Scotland.
Responding to those calls, chair of the inquiry Baroness Heather Hallett said that "everyone knows [it] is not a decision for me".
"I can promise, however, that the UK inquiry will do its utmost to investigate and analyse fully and fairly the most significant issues that concern the people of Wales," she said.
The Welsh government has previously argued that decisions taken in Wales should be considered within a UK context.
Baroness Hallett said it was "probably appropriate that we begin these hearings in Wales in a week that includes St David's Day but also includes the National Day of Reflection on Sunday, when we remember those who lost their lives during the pandemic".
As the hearing got under way on Tuesday, a 20-minute impact film, with contributors speaking in both English and Welsh, was played.
Baroness Hallett described it as "extremely moving" and "it reminds us all why we are here".
Image:Baroness Heather Hallett
Image:Baroness Heather Hallett
'Adding confusion'
The inquiry was told that in March 2020, Welsh Labour decided not to hold its conference in Llandudno in-person, due to the emerging threat of the virus.
Lead counsel to the inquiry, Tom Poole KC, said Labour Senedd member, Lee Waters, said in a WhatsApp message: "I do think that it's an odd signal to send that we're cancelling conference, but allowing 70,000 to gather in Cardiff on Saturday".
The message was in reference to the Six Nations match between Wales and Scotland due to be held in Principality Stadium on 14 March 2020.
It was called off by the Welsh Rugby Union on 13 March, but not before 20,000 Scottish rugby fans had travelled to the Welsh capital, the inquiry heard.
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On 24 April, then Wales secretary, Simon Hart, wrote to the Welsh government saying its framework for recovery - which outlined how restrictions would be eased - "did not mention the UK government once".
Mr Hart stated that "unless the evidence being relied on by the Welsh government to diverge from a UK-wide plan is explained, then the Welsh government will be guilty of adding confusion to an already challenging period of recovery".
'Treasury for England'
The successor to the coronavirus job retention scheme was due to start on 1 November 2020.
The Welsh government asked the chancellor for the plan to start earlier in Wales after it announced a firebreak lockdown in the country.
Mr Drakeford, in written evidence to the inquiry, described the rejection of that request as "one of the most misguided decisions of the whole pandemic".
Mr Drakeford said the impact of that decision was that the Treasury was "in effect acting as a Treasury for England, not a Treasury for the UK".
Mr Poole told the inquiry that this is denied by the UK government, including by the Prime Minister Rishi Sunak, who was chancellor at the time.
