Boult was ruled out of the final Test after fracturing his right hand when he was struck on the gloves while facing a Mitchell Starc delivery in their 247-run defeat in Melbourne. He did not bat in the second innings.
New Zealand coach Gary Stead said 35-year-old Somerville, who has played three Tests and picked up 14 wickets, was called up instead of another paceman as the conditions at the Sydney Cricket Ground were conducive to spin.
“There’s no secret the SCG pitch is one of the more spin-friendly in Australia,” Stead said in a statement. “Will offers something different to our two other spinners in the squad with his right arm off-spin and height.
“The fact he’s played a lot of cricket in Sydney during his career for New South Wales will also be helpful as we prepare for this final Test.”
Australia have already wrapped up a 2-0 victory in the three-match series after winning the first Test in Perth by 296 runs.