Born in Newcastle, New South Wales, Southwell played junior rugby league for the Kotara Bears. At age 13, she gave up rugby league as she wasn't allowed to play anymore due to her age.[1]
Southwell signed with Newcastle Jets in 2014.[3] On 20 September 2014, she made her debut in a match against Perth Glory, becoming the league's youngest goalkeeper at the age of 15.[4][5][6] She made six appearances for the team during the 2014–15 W-League season. Newcastle finished in fifth place during the regular season with a 5–5–2 record.[6] During the 2015–16 W-League season, Southwell made 12 appearances for Newcastle and helped the team finish in sixth place with a 3–5–4 record.[6]
In 2018, Southwell returned to rugby league, joining CRL Newcastle in the NSWRL Women's Premiership.[12] In June, she represented NSW Country at the Women's National Championships.[13]
In May, Southwell represented NSW Country at the Women's National Championships.[19] In July, she joined the Sydney Roosters NRLW side.[20] On 21 June, she started at lock for New South Wales in their 14–4 win over Queensland.[21]
Southwell was a member of the Sydney Roosters team that won the premiership in the postponed 2021 NRL Women's season, playing in all seven matches including the Grand Final victory on 10 April 2022.