Emma Kane is currently chief executive of Redleaf Communications, which will be combined with the UK arm of Newgate.
Brian Tyson is managing partner of Newgate Australia and will continue to run that business while Kane and Tyson will together manage the international network as joint CEOs.
Steffan Williams, the previous group chief executive, has left with immediate effect.
John Foley, chairman, said: "Redleaf has been a great acquisition for Porta and it now makes absolute strategic sense to combine the business with Newgate and create an enlarged business to capitalise on the expertise and proven track records of the exceptional people at both firms.
"In Australia, from start-up five years ago with a core group of 8 people in offices in Sydney and Melbourne, the business now has more than 100 professional staff and opened additional offices in Canberra, Brisbane and Perth.
"Together, Brian and Emma will give us the leadership we need on both sides of the world to help roll out an integrated offering in our major markets.”