"The end of the physical conflict was of course only the beginning," said Rankin, stressing that the next tasks are "accountability that arose from events during the conflict and to achieve lasting reconciliation between Sri Lanka's communities."
He also cited progress in such areas as infrastructure development in the war-torn areas and resettlement of many war-displaced people.
Australia and Britain have pushed for engagement with Sri Lanka rather than isolating it and have encouraged countries to participate in the Commonwealth leaders' meeting in Sri Lanka despite the calls for the boycott.
Last week, Canadian Prime Minister Stephen Harper said he would not attend due to human rights concerns and he threatened to cut off funding to the Commonwealth group. Harper has accused Sri Lanka of failing to uphold the Commonwealth's core values.
Sri Lanka has rejected Harper's comments, saying "he has his own political obligations."
The Commonwealth is a loose association of 54 members, mostly former British colonies, and the leadership summit is biennial.