Rees was found guilty of making contact with the eye/eye area of Hartley after half an hour of the Blues' second successive European loss to the Saints.
Rees pleaded not guilty to the complaint on the basis that any contact had been accidental.
The independent judicial officer determined that the 27-year-old was guilty of foul play but found that the contact had been reckless rather than intentional.
Rees' actions were deemed to be at the low end of the level of seriousness for an offence of this type and, having taken into account all aggravating and mitigating factors, imposed a suspension of 12 weeks.
That means Rees cannot play again until Rees March 31, 2011, 12 days after Wales finish their Six Nations campaign against France in Paris, although both parties have the right to appeal.
Rees was expected to challenge Lions star Mike Phillips for the Wales No9 jersey this spring as he aimed to build on his nine international caps so far.
His absence could open the door for 2005 Lion Dwayne Peel but his recent admission from Welsh player testing at the Vale of Glamorgan may mean his return remains unlikely.