Ms Maina's husband, Cyrus Bernard Maina Njuguna, and another man, John Njuguna Waithira, were arrested over Thursday's shooting in the exclusive suburb of Karen, the ABC reports.
Her lawyer, George King'ori, earlier on Tuesday told AAP the couple was in the midst of divorce proceedings in the Kenyan courts after the 40-year-old had been granted a restraining order against her husband in April.
A post-mortem examination confirmed Ms Maina, originally from the NSW town of Armidale, was killed by a single gunshot wound, the ABC reports.
Mr King'ori said the mother-of-two may have been on her knees when she was shot.
The 40-year-old moved to Nairobi from Sydney in 2015 so her two sons could experience another culture and grow up near her husband's extended family, a friend told AAP this week.
Before moving to Nairobi, where Ms Maina was head teacher at the elite Hillcrest Preparatory School, she taught English and drama for almost five years at Sarah Redfern High School in Sydney's southwest.
Australia's foreign affairs department is in contact with Kenyan authorities responsible for the investigation into Ms Maina's death.
It is also providing consular assistance to her family in Kenya and Australia.
