His comments come after an article in The Times on Friday revealed that Oxfam workers had employed beneficiaries as prostitutes while working in Haiti in the wake of the 2010 earthquake and the charity had failed to tell the Charity Commission or the government about the exact nature of the affair.
NCVO was joined by the charity leaders group Acevo and the international development charity umbrella body Bond in warning that the sector needed to learn the lessons of the Oxfam case in order to protect both beneficiaries and trust in the sector.
Kellner said: "International development charities take abuse issues very seriously and are constantly working to root out those who are looking to do wrong under the guise of helping some of the world’s most vulnerable."
He said all major charities had policies and procedures in place to minimise the risk of abuse taking place.
"However, it is clear that more can and must be done, not only to revive the reputations of those charities in the news, but also to maintain public confidence in the sector as a whole," he said.
Kellner welcomed Oxfam’s announcement that it would make a package of changes to its safeguarding and recruitment policies in the wake of the scandal to prevent a repeat of the "disgusting behaviour" reported.
"These reports should and I believe will prompt other organisations to ensure that they, too, enforce the very highest standards."
He said the public would be deeply upset by the allegations and the NCVO would work alongside Bond, the commission and the Department for International Development to ensure overseas charities were following the highest standards
"We must make sure that bad behaviour, however rare, is never tolerated," he said.
Tamsyn Barton, chief executive of Bond, said the reported behaviour of Oxfam employees was "completely deplorable" and contrary to the values of the development and humanitarian sector.
"It is very important that we learn the lessons from this as a sector to ensure that we have the best preventive mechanisms in place," she said.
"However, we cannot allow the actions of a few to undermine the incredible work charities do to provide shelter, food and healthcare in some of the most difficult environments in the world, and often at desperate times for the world's poorest people. These organisations are often the only lifeline for people facing conflict, climate change and extreme poverty."
She said Bond was coordinating with its members and the Charity Commission to ensure robust safeguarding and whistle-blowing policies were in place.
Kristiana Wrixon, head of research at Acevo, said that although these allegations had come from an international development charity, they raised questions for the entire charity sector about ensuring that safeguarding was a priority.
"We need to recognise that we need to be sure safeguarding is a priority for us and, when things go wrong and mistakes happen, we do everything to reduce the risk that this will happen again," she said.
"Sexual abuse and misconduct is endemic across all countries and all sectors in our society. As we work towards more transparency, as everybody wants to do, we might have more cases of abuse or exploitation being exposed, so we need to demonstrate that we have done everything we can to protect beneficiaries and make sure we have the best policies possible in place."