A North Korea defector group this week twice defied a ban by Seoul on flying anti-Pyongyang leaflets across the heavily fortified Demilitarised Zone dividing the peninsula.
The launches by the Fighters for a Free North Korea were the first since the law was passed in December.
The group "flew 500,000 leaflets, 500 books and $5,000 in cash bills distributed between a total of 10 large balloons over two occasions near the DMZ between April 25 and 29", said its chairman Park Sang-hak.
Activist groups have long sent flyers critical of North Korean leader Kim Jong-un over human rights abuses and his nuclear ambitions across the DMZ, either flying them by hot air balloon or floating them across rivers.
The leaflets have infuriated Pyongyang, which issued a series of vitriolic condemnations last year demanding Seoul take action and upped the pressure by blowing up an inter-Korean liaison office on its side of the border.
The South Korean parliament rapidly passed a law criminalising sending leaflets and USB drives - a favoured method of distributing information and entertainment.
Under the measure, those convicted of sending leaflets face a maximum penalty of three years in prison or a fine of 30 million won (€22,000).
The law has raised concerns over freedom of speech, with the US - a treaty ally of the South - describing it as a "significant human rights issue" in an annual report in March.
North Koreans "had the right to know the truth even though their rights as human beings are taken away by the regime", Mr Park said, criticising the South's "gag" as "the worst law".
Both Koreas used to regularly send leaflets to the other side but agreed to stop such propaganda activities - including loudspeaker broadcasts along the frontier - in the Panmunjom Declaration signed by Mr Moon and Mr Kim and at their first summit in 2018.
But civilian groups in the South, mainly led by defectors, continued their activities, raising fears of retaliation among locals living along the frontier.
Seoul's Unification Ministry handling inter-Korean relations said the leaflet law "was for the safety and lives of residents in border areas".
In response to the latest launches, it said, authorities "will take appropriate measures in accordance with the spirit of the law once the facts are established".