In January 2022, he was named as the captain of Sri Lanka's team for the 2022 ICC Under-19 Cricket World Cup in the West Indies.[6] In Sri Lanka's opening fixture of the tournament, he took a five-wicket haul,[7] and was named the player of the match.[8] In Sri Lanka's next match of the tournament, he took another five-wicket haul, and was again named the player of the match.[9] In Sri Lanka's Super League playoff semi-final match against South Africa, Wellalage scored 113 runs, with Sri Lanka winning the match by 65 runs.[10] He became the first captain of the Sri Lankan team to score a century in the Under-19 Cricket World Cup.[11] He finished the Under-19 World Cup as the leading wicket-taker, with seventeen dismissals.[12]
In April 2022, Sri Lanka Cricket (SLC) named him in the Sri Lanka Emerging Team's squad for their tour to England.[13] On 25 May 2022, during the tour of England, he made his Twenty20 debut, against Surrey.[14] The following month, he was named in the Sri Lanka A squad for their matches against Australia A during Australia's tour of Sri Lanka.[15] Later the same month, he was named in Sri Lanka's One Day International (ODI) squad, also for their series against Australia.[16][17] He made his ODI debut on 14 June 2022, for Sri Lanka against Australia.[18] The following month, he was added to Sri Lanka's Test squad for the second match against Australia.[19]
In July 2022, he was again named in Sri Lanka's Test squad, this time for their home series against Pakistan.[20] He made his Test debut on 24 July 2022, for Sri Lanka against Pakistan.[21]
On August 2, 2024 first ODI match against India Dunith scored his maiden ODI half century. He scored unbeaten 67 runs from 65 balls hitting seven boundaries and two sixes. He also took two crucial wickets of Rohit Sharma and Shubman Gill while bowling. Finally the match ended as a tie. Due to his performance he won player of the match award.[24] He made his T20I debut on 15 October 2024 against West Indies.
Chamika Karunaratne was named as a reserve player. Chamika Karunaratne replaced the injured Dasun Shanaka. Angelo Mathews and Dushmantha Chameera were named as travelling reserves. Angelo Mathews and Dushmantha Chameera replaced the injured Matheesha Pathirana and Lahiru Kumara.