On 15 February 2014, at the USA Women's Sevens, Nievas took charge of a match between Australia and China. This was her first match in charge on the World Rugby Women's Sevens Series circuit. On 30 November 2017 at the Dubai Women's Sevens she took charge of a match between Russia and England. This was her 100th World Rugby Women's Sevens Series match. In 2014 Nievas made her international debut as a referee in the women's fifteen-a-side game when she took charge of a game between New Zealand and Samoa at Eden Park. She subsequently refereed games in the Women's Six Nations Championship and at both the 2014 and 2017 Women's Rugby World Cups. She also refereed the women's final at the 2016 Summer Olympics.