He is primarily known for portraying characters in epic fantasy works, such as Crixus in the TV series Spartacus, Allanon in The Shannara Chronicles, Slade Wilson / Deathstroke in Arrow, and Azog the Defiler in The Hobbit trilogy.

In an audience Q&A in Boston, Bennett talked about the car accident he experienced at a young age that killed his mother and brother and put him in a two-week coma.

Bennett then starred alongside Claudia Karvan in the miniseries The Violent Earth, produced by French company Gaumont.

In 1999, Bennett landed his first leading role in a feature film named Tomoko, shot on location in Tokyo, starring opposite Rumiko Koyanagi.

Bennett returned to New Zealand in 2000 to appear in the popular television drama, Shortland Street, then went on to play a cop turned lawyer in Street Legal before working with his director cousin Michael Bennett on the Māori Twilight Zone-styled series Mataku before working with Richard Taylor from the Academy Award-winning firm Weta Workshop on Creature Quest.

In August 2002, Bennett landed a screen test opposite Angelina Jolie for a part in the Tomb Raider sequel The Cradle of Life.

In May 2014, Bennett revealed he had lost two roles in Stargate Atlantis and the 2011 film Conan the Barbarian to actor Jason Momoa.