The agencies were appointed following a pitch process and will work on a campaign to run across TV, digital and social channels.
Fairfx.com, which competes with the likes of Travelex and the Post Office, previously created its advertising in-house.
Ian Stafford-Taylor, the chief executive of Fairfx.com, said: "MBA and PHD impressed us with their insight and knowledge of our sector as well as their creative and media talent.
"We are looking forward to seeing the results as we enter the main holiday season."
The brand has a positioning of cutting out the middlemen in foreign agency exchange and using technology to move money swiftly to customers.
Stephen Maher, the chief executive of MBA, said: "We are thrilled to be working with Fairfx.com and looking forward to helping them become the 'go to' brand in this space."
Hugh Cameron, the chief strategy officer at PHD, said: "Fairfx.com is a revolutionary brand with a real competitive advantage and we cannot wait to help them realise that potential."