Born in Christchurch on New Zealand's South Island, Seiuli moved to the town of Timaru, which lies roughly halfway between Christchurch and Dunedin, in 2002 and attended Timaru Boys High School. He captained the school's first XV rugby side in his final year before going on to play once for South Canterbury in the Heartland Championship. He later moved to Otago and turned out for the Taieri Eels with whom he won the Dunedin Premier Club Championship in 2012.
Success in local club rugby saw Seiuli earn a spot in the Otago squad for the 2012 ITM Cup. After debuting in a match against local rivals Southland in week three of the competition, he went on to play seven times throughout the year as the Razorbacks reached the Championship final. Seiuli was not named in any of the five New Zealand Super Rugby franchise's squads ahead of the 2014 Super Rugby Season, however an injury crisis in the Highlanders front row saw him called into their set-up to provide cover.
He once again found himself in the Super Rugby wilderness in 2015 before injuries once again provided him with an opportunity in 2016. He finally made his Super Rugby debut in an impressive 26-13 victory over the Chiefs in Hamilton on 7 May 2016 and went on to play seven times in total during the year as his side reached the semi-finals before going down to the Lions in Johannesburg.
He was included in the Highlander 2017 Super Rugby squad and featured in 14 of the Highlanders 16 Super Rugby matches, whilst also coming off the bench against the British and Irish Lions. Having missed the 2019 Super Rugby season due to an ACL injury, Seiuli returned to action for Otago in August before he signed a two-year deal with Glasgow Warriors in September 2019 to join the club following Otago's 2019 ITM Cup campaign. In total he made a combined 69 appearances for Highlanders and Otago scoring four tries.
Seiuli moved to Scotland and made his Guinness PRO14 debut for the Warriors on 30 November 2019. He was nominated for the Glasgow Warriors Player of the Season award in 2019-20 and went on to make 29 appearances and scored two tries in the Scottish capital. On 20 July 2021 Seiuli joined the Dragons ahead of the 2021/2022 United Rugby Championship campaign.