Oxfam's corporate sponsors are reviewing their relationship with the company and the charity is also under investigation by the United Kingdom government's Charity Commission agency.
She had previously travelled to countries like Cambodia and Thailand to promote Oxfam's work.
The charity group's name took another hit on Tuesday when Oxfam International's chairman Juan Alberto Fuentes Knight was arrested in his native Guatemala over government graft allegations unrelated to the sexual assault claims.
"As programme director at the time, I am ashamed that this happened on my watch and I take full responsibility", she said, adding that she was "desperately sorry for the harm and distress that this has caused to Oxfam's supporters, the wider development sector and most of all the vulnerable people who trusted us".More news: Big Issue man George's royal appointment with Harry and Meghan
More than 1,000 people cancelled their regular donations to Oxfam over the weekend as more claims about a sexual misconduct scandal emerged.
Oxfam's internal investigation into the use of prostitutes in Haiti led to the dismissal of four employees and three others being allowed to resign, including van Hauwermeiren.
The investigation was launched a day after Oxfam Great Britain's deputy chief executive resigned and apologised to the government and donors.
Scandal hit Britain's best-known charity after media reported how Oxfam had covered up the use of prostitutes by senior staff after the 2010 Haiti quake.
Ms Mordaunt is due to give a speech to representatives from global aid agencies and ministers from over 20 countries at the Agenda 2030 for Children: End Violence Solutions Summit. "We will consider any further action needed as a result". It hurts me very deeply.More news: Asma Jahangir: Her life in pictures
Corporate sponsors like Visa and M&S said they were "monitoring" the situation, according to the Telegraph. "But people chose to abuse and exploit vulnerable adults and younger children and there was a failure to deal with that".
Following Oxfam's sexual misconduct scandal, the government is expected to tell aid agencies that funding will be withdrawn if they can not show they are preventing abuse by staff.
He said: "We expect our partner organisations wherever they are to monitor their work closely, and to be open, honest and transparent with us, and the public whenever there are any suggestions of abuse or malpractice".
Meanwhile, the Oxfam's ambassador, actor Minnie Driver, resigned from the post.
Former aid worker Amira Malik Miller told Irin she raised concerns about van Hauwermeiren and several colleagues to Merlin, which merged with Save The Children in 2013.More news: While Forces Expected Strike In Kashmir, Fidayeen Struck Jammu Camp