Navi Pillay, who has previously called for a war crimes investigation against Sri Lanka, said she was only holding it to human rights standards agreed to by all nations.
"I am not writing my own statute. I am looking at the (human rights) framework that was also developed by Sri Lanka," she told reporters as she began her first meeting with local UN staff in Colombo.
"If I raise criticism, it is on whether they (Sri Lanka) comply with those standards," she added. "I have not come to criticise. I have come to raise human rights concerns."
Her week-long mission will include talks with President Mahinda Rajapakse and visits to the former war zones in the north and east.
The government's U-turn in allowing her free access came as Canada leads calls for a boycott of a Commonwealth summit scheduled to take place in the Sri Lankan capital later this year.
A pro-government group said it will hold a demonstration outside the UN offices in Colombo on Monday to protest at Pillay's visit. The same group has held similar protests in the past and called Pillay a US stooge.
Sri Lanka has resisted pressure from the UN and Western nations for an investigation into allegations that up to 40,000 civilians were killed in the final months of its separatist war, which ended in 2009.
A no-holds-barred military offensive crushed Tamil Tiger rebels who at the height of their power controlled a third of Sri Lanka's territory. Rajapakse has since been dogged by claims of indiscriminate killing of ethnic Tamils.
During her visit, Pillay is scheduled to hold talks with local rights activists.
"We are in the process of finalising our memo to her. We want to talk about the culture of impunity during and after the war," activist Nimalka Fernando told AFP.
"We are also specifically taking up the issue of media freedom in Sri Lanka."
Fernando said an armed break-in at the Colombo home of a senior journalist at the Sunday Leader newspaper on Saturday could be linked to her work, although police insisted it was only an attempted robbery.
The attack was the latest of several violent incidents involving the staff of the privately-run newspaper, whose founding editor Lasantha Wickrematunge -- a fierce government critic -- was shot dead while he drove to work in January 2009.
"The murder of the Sunday Leader editor has still not been solved and this is also something that we will take up," Fernando said.
Tamil groups are banking on Pillay's first visit to Sri Lanka to revive calls for a war crimes probe.
"We will take up with her the question of accountability, the issue of thousands of missing people, the militarisation of Tamil areas and the lack of political freedoms," Tamil National Alliance lawmaker Suresh Premachandran told AFP.
Pillay's visit follows two resolutions by the UN Human Rights Council (UNHRC) in as many years demanding Colombo hold an independent investigation into "credible allegations" that troops shelled hospitals and refugee camps, and executed surrendering rebels.
The government insists that its troops did not kill civilians and has slammed the UNHRC for its "ill-timed and unwarranted" resolutions.
The government's change of heart in welcoming the rights chief could signal a desire to improve its image before a UNHRC session in September and the Commonwealth Heads of Government Meeting in November.
"She has not accepted what we have done (to improve the rights situation)," Sri Lanka's human rights envoy to the UN, Mahinda Samarasinghe, told reporters in Colombo last week.
"So we are showing her what we have done and we are also allowing her to visit anywhere and meet anyone."
Until recently, the government declared much of the former northern war zone off limits to foreign journalists, aid workers and even UN staff.
In the past Samarasinghe, who is also the plantations minister, has criticised Pillay for lacking "objectivity and impartiality" and working outside her mandate.