"I will appoint a presidential commission next week," Sirisena said.
"It has been a month since we formed the government, I ask myself why the delay in investigating corruption," said the 63-year-old leader at a gathering in North Central Province.
Crackdown on corruption and misuse of public funds were major planks Sirisena campaigned on ahead of the bitterly- fought January 8 presidential poll, in which he dethroned Rajapaksa after his nine-year rule.
"Black money invested in various foreign countries is going to be investigated," cabinet spokesman Rajitha Senarathne had said, referring to illegal capital flight.
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Rajapaksa has denied all the allegations against him.
Sirisena, in his election manifesto, had pledged a 100-day programme to set up independent commissions to suggest ways to free key governance sector of political interference.
The government estimates around USD 5.31 billion were taken out of the country in 2013 alone.
The need for a presidential panel arises as the existing Commission to Investigate Bribery or Corruption is viewed weak and lacking resources for effective anti-corruption crackdown.