More than £60,000 has been lost to scammers in recent days, police in Northern Ireland said.
One victim transferred just over £20,000 to a cloned site and it was only when he tried to make another transfer that he was alerted by his bank.
PSNI Chief Superintendent Simon Walls said: “Unfortunately, we are again dealing with people and businesses that have been left to deal with the terrible consequence due to the unscrupulous actions of criminals.
“It is despicable and in both of these reports, substantial sums of money have been taken from people acting in good faith.”
On May 4, a report was made to police that a business in Tyrone had been duped in a mandate fraud where an email was sent to the company asking for payment to be sent to a new bank account.
A sum of just over £20,000 was transferred to the bogus account.
On May 5, a report was made that a woman living in Co Antrim had received a call telling her she was wanted by police abroad.
She was duped into transferring £20,000 to accounts.
Mr Walls said the most recent attacks by fraudsters show scammers’ continued sophistication and intent.
He added: “I can’t stress enough to never, ever give out any personal or financial information to an unverified source via email or over the phone.
“If a supplier suggests they have changed an account number please ensure you are 100% sure this is the case. Don’t take an email as proof.
“Please, be vigilant, and be on your guard.”