Pompey required a late leveller to ensure their dominant display resulted in a draw at Accrington.
Ronan Curtis' strike had given Danny Cowley's men an early lead, but goals from Harry Pell and Matt Butcher flipped the contest on its head.
The Blues created a hatful of chances, however, and Harness turned home a John Marquis point to ensure the contest finished in a 2-2 stalemate.