The Serious Fraud Office says it is possible the alleged fraud by an employee of the Hibernian Credit Union could be more than $1 million.
The Hibernian Catholic Benefit Society is a friendly society which offers financial services to about 2800 members, of which about 20% to 30% are also with the credit union.
Serious Fraud Office director Adam Feeley says investigators are trying to work out the scale of the alleged fraud but it appears to be certainly in the six figures, or even a seven-figure sum.
Mr Feeley says at this point, they believe it is just one person involved but can't yet rule that out, but are expecting the investigation to be swift.
The president of the society, Michael McBride, says the board is trying to make the losses as light as possible for members.
He says it's only the credit union that's affected, and the society, which offers financial services like mortgage finance and life assurance, remains in a very strong position.