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."

A Royal Navy Frigate and submarine are also thought to be involved in the search, along with Canadian and French maritime patrol aircraft.
The Telegraph reports the French plane has searched for the submarine for at least 10 days.
The RAF currently has no maritime patrol aircraft of its own.
But according to the BBC's defence correspondent Jonathan Beale, this lack of patrol aircraft is expected to be addressed in Monday's Strategic Defence and Security Review.
The submarine was detected north of Scotland. The Ministry of Defence did not confirm it was looking for a foreign submarine.
"We can confirm that allied maritime patrol aircraft based at RAF Lossiemouth for a limited period are conducting activity with the Royal Navy," a statement said.
"We do not discuss the detail of maritime operations."
There have been previous concerns that Russia could be developing plans to have submarines sever key internet communications during future wars, following a spike in its naval activity near the locations of undersea cables.
Norton have signed the Australian duo of Cameron Donald and David Johnson for the 2016 Isle of Man TT races.
[NEXT_CONCEPT]
An RAF plane is "conducting activity" off the Scottish coast, the Ministry of Defence says, amid reports of a Russian submarine being spotted in the area.