Speaking on BBC One's Andrew Marr Show this morning, Ms Mordaunt said she had only learned of the allegations against the charity when the Times reported them on Friday - and that she will hold urgent crunch talks with Oxfam's management tomorrow.
Thomson did not mention Sunday's report in The Observer, The Guardian's Sunday newspaper, that alleged that the aid group also covered up allegations that staff in Chad paid for sex with young women.
United Kingdom officials have said that Oxfam needs to hand over all its information on the issue in order to not lose funding.
Oxfam announced a new raft of measures to tackle sexual abuse cases after being ordered to meet the British government on Monday to explain its handling of a 2011 prostitution scandal involving its aid workers in Haiti.
The former Secretary of State said that "the reason why Oxfam has landed in this position is because they have not been fully open and transparent about what happened".
Oxfam said it now had a dedicated safeguarding team, a confidential whistleblowing hotline and safeguarding contact point within countries, and a code of conduct that stipulated: "I will also not exchange money, offers of employment, employment, goods or services for sex or sexual favours".
The worldwide development secretary is meeting the charity on Monday and warns it can not remain a "partner" if it fails to account for its actions, reported BBC.More news: Sunjwan camp attack: One more terrorist killed
Mark Goldring said the UK-based charity was ashamed of what it had got wrong but had taken action and been proactive about going public on the matter.
Oxfam's boss say he is "deeply ashamed" over accusations its staff used prostitutes in Haiti in the aftermath of the 2010 natural disaster.
She said that as a direct result of the Times story, staff members had raised concerns about how employees in Haiti were vetted and recruited. I made this our own agenda, I did my research, this [sexual abuse] is well-documented.
The charity added it will soon deliver a "safeguarding training course for faith organisations, especially churches, so they can understand, recognise and respond to safeguarding issues, and develop a safer culture".
British charity Oxfam says it will strengthen its internal safeguards in response to allegations staff members working in Haiti following the devastating 2010 quake engaged in sexual misconduct.
"It is utterly despicable that sexual exploitation and abuse continues to exist in the aid sector", Ms Mordaunt said.
Oxfam has denied allegations that it tried to hide that some of its staff paid prostitutes in the aftermath of the quake.
During the probe, Oxfam dismissed four staff members and another three resigned, including van Hauwermeiren.