It has been reported by the British newspaper that a deal has been agreed between the two Championship clubs.
Reading were also reportedly interested in signing Mackie on a permanent contract this summer, but they failed to reach an agreement with Forest.
The attacker could score just four goals and managed to create only 32 chances in 45 Championship appearances in the 2013-14 season.
The 29-year-old had a pass accuracy of 75% and a shot accuracy of 55%, and won 28 headers, 37 dribbles and 54 tackles.
The Scotland international was sent out on loan to Championship rivals Reading for the 2014-15 campaign.
Mackie scored five goals and created 24 chances in 32 league appearances for the Royals this past term.
The former Milton Keynes Dons man had a pass accuracy of 74% and a shot accuracy of 50%, and won 30 headers, 27 dribbles and 24 tackles.