The government's ratification of the Optional Protocol to the Convention against Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment is welcome but the inspection regime must include Australia's offshore detention system, Greens Justice Spokesperson Nick McKim says.

"The government's decision to ratify OPCAT is an important and welcome human rights decision," Senator McKim said.

"It has the potential to improve conditions in, and oversight of, prisons and places of detention across Australia."