India's Prime Minister Manmohan Singh and Canada's Prime Minister Stephen Harper say they won't attend the three-day conference, which begins in the Sri Lankan capital, Colombo, on Friday over concerns about human rights abuses.
Sri Lanka is accused of committing war crimes, particularly during the final months of the civil war in 2009. That has prompted calls for leaders to boycott the meeting and oppose Sri Lanka as chair of the Commonwealth.
But Mr McCully says Sri Lanka will be opened up to international scrutiny by virtue of countries attending the meeting.
He says the Government will still be raising human rights issues with Sri Lanka, and he himself will tour the north of the country to see what progress has been made since the end of the civil war against Tamil rebels.
Opposition parties in New Zealand say, at the least, New Zealand should go on record as opposing Sri Lanka being made the Commonwealth chair.
Prime Minister John Key said on Monday that he would still go to Sri Lanka because New Zealand believes it is more constructive to engage. However, he said he would directly raise concerns with Sri Lanka's president about historic allegations and more recent incidents.
Mr Key said Sri Lanka is made Commonwealth chair automatically with the country's hosting of this year's CHOGM.
Green Party MP Jan Logie who was detained in Sri Lanka on Sunday says a strong message needs to be sent about the country's human rights' abuses.
Ms Logie told Radio New Zealand's Morning Report programme what happened to her is symbolic of a country that has no freedom of speech and where land grabs and constitutional changes are causing further repression.
The MP was in Sri Lanka to talk to people pushing for an independent investigation into accusations that government forces committed war crimes in 2009. Immigration officials shut down a news conference called by her and an Australian Greens senator before taking their passports and holding them in a hotel for three hours.