Both men will contest the opening Superbike and Senior TT races on the SG5 Norton machine.
Donald, a two-time winner who has represented the team since 2014, said Norton have made a "huge amount of progress in that time".
Fellow countryman Johnson, who made his TT debut in 2010 winning the Newcomers Trophy, is a rising star of the sport.
The Adelaide racer said: "It is going to be an incredible experience riding a Norton around the Isle of Man Mountain Course, and with Cameron Donald as my team-mate is a massive honour.
"I'm really stoked and will give it my best shot."
Stuart Garner, chief executive of Norton Motorcycles, added: "We've made incredible progress since taking Norton back to the TT.
"This was always going to be a long-term project and we're definitely on the next stage of our evolution. Running a two-man team will give us the best opportunity to move things forward."
Norton have signed the Australian duo of Cameron Donald and David Johnson for the 2016 Isle of Man TT races.