The judging panel said Ms Arnold was "a champion of young girls and women in Cambodia, working to bring them education and work experience".
Ms Arnold, who established the trust, has also worked with lawyers in Cambodia to help give them a voice within the international legal community and to enforce change.
Specialist intellectual property lawyers James & Wells, which has an office in Tauranga, won the Canon Intellectual Property Law Award.
Tauranga-based barrister Kate Barry-Piceno was also a runner-up for the Resource Management and Environmental Law, Tauranga firm Sharp Tudhope was a runner-up for the Hays Employment Law Award, and Tauranga firm Mackenzie Elvin was a finalist in the Family Law Award.