Shepherd, who won nine caps for the Wallabies, can also play full-back and will offer Saints director of rugby Jim Mallinder an additional goal-kicking option.
The 28-year-old is the leading points scorer for the Western Force and will move to Franklin's Gardens at the end of the Super Rugby season.
"Cameron is a very talented player who has shown regularly that he has the ability and temperament to compete at the highest level," said Mallinder.
"He has a positive impact for his team's attacking capability but is also sold defensively and under the high ball. His versatility and quality will give us added depth in the back three positions.
"We're looking forward to welcoming him to Franklin's Gardens and believe that he will contribute a good deal to the squad."
Ashton is leaving Premiership semi-finalists Northampton to join Saracens.
Shepherd said: "It's an exciting opportunity for me.
"Jim has got the team playing a great brand of rugby which I believe suits my style of play and to which I think I've got a lot to offer.
"It is definitely going to be a new challenge for me. The Premiership is a tough competition and there are a lot of teams which like to play exciting rugby, especially the Saints.
"You can see that with the quality of player that there is at the club, players like Ben Foden and George Pisi, who are established internationals."
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