The Federal government has today confirmed it is spending $20 million building a new immigration jail in Papua New Guinea to open by the end of next year.

"The Australian government is spending millions of dollars on new offshore jails, all the while claiming that their policies are working," Greens Immigration spokesperson Senator Nick McKim says.

"This is on top of the $4 billion plus the government has confirmed has been spent on Manus Island and Nauru in the last three years."