In 2009, Swiss brokerage Helvea, estimated Greek holdings in Swiss banks at around 19.7 billion euros ($26.6 billion), 99 per cent of which was undeclared.
Greece struggling with tax clampdown
Despite tightening tax collection since its first bailout in 2010,Greece is still struggling to clamp down on tax evasion, both international and domestic.
"As long as tax laws are constantly changing and the respective practice is all but certain, it will be very difficult to repatriate funds," said Anna-Maria Widrig Giallouraki, a senior manager at PwC in Zurich, Switzerland.
(Read more: Why hedge funds are wary of Greece)
In 2010, a list of more than 2,000 Greeks with bank accounts in HSBC's Swiss branch was handed over to the Greek authorities by Christine Lagarde, then a French cabinet minister. Coined the "Lagarde list", it triggered a public outcry when it was published by Greek media in 2012.
Last year, Greece indicted the first recipient of the list, former Greek Finance Minister George Papakonstantinou, of hiding evidence and deleting two names of his relatives. He is due to go on trial.
Follow us on Twitter: @CNBCWorld