Sosene, who won the Māngere electorate last year, will undergo surgery and then remain on leave for recovery, Labour said in a statement.
"I wish Lemauga all the very best for her surgery and recovery," Labour leader Chris Hipkins said.
"Receiving a diagnosis like this is never easy and is particularly hard on whānau. We in Labour are right behind Lemauga and will support her in any way she needs during this tough next period.
"Lemauga and I discussed what she would like me to say on her behalf, and it is to encourage women of all cultures to get checked. You never think it will be you, or someone close to you - but cancer can affect so many people quietly.
"On behalf of Lemauga's family, thank you for your best wishes and prayers. Fa'afetai lava."
During Sosene's absence, Jan Tinetti will take over as the party's spokesperson for internal affairs, while deputy Carmel Sepuloni picks up the associate roles covering Pacific peoples, social development and employment.
Greens' co-leader Marama Davidson is also currently on leave after receiving a breast cancer diagnosis earlier this year.
She shared an update on social media over the weekend: "Just popping in briefly to thank everyone for the overwhelming and beautiful aroha that you all keep sending through to my whānau and I. It means EVERYTHING. I send my love back to everyone with my full gratitude and appreciation."
