Lomax debuted for the Māori All Blacks in November 2017 against Canada and played a total of 6 games for the side between 2017 and 2019. Lomax became All Black number 1180 making his debut against Japan in Tokyo in November 2018, but did not play for the side again until 2020 where he made 5 appearances.
After poor performances in Super Rugby, Lomax was dropped from the All Blacks and made his debut for the Māori All Blacks against Ireland in 2022. Lomax was re-called to the national side for the 2022 Rugby Championship and became a regular starter for New Zealand during this time. Lomax was picked for the 2023 Rugby World Cup and started in the Final, an 11–12 loss to South Africa.