Despite losing a seamer, New Zealand have included the 35-year-old spinner for the dead rubber considering the nature of the surface at the SCG which traditionally favours spin.
Coach Gary Stead said the inclusion of Somerville was a nod to the expected conditions in Sydney.
“There’s no secret the SCG pitch is one of the more spin-friendly in Australia,” said Stead.
“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,” he added.
New Zealand have already lost the series, facing crushing defeats in the two matches played in Perth and Melbourne.