"The FSB believes that the fine of R150 000 or 36 months in prison does not acknowledge the extent of the damage caused by Mr [J Arthur] Brown's actions," spokesperson Tembisa Marele said.
The National Union of Mineworkers (NUM) and Cosatu echoed these sentiments.
"The union is highly disappointed at the light sentence and the discrepancies in judgments passed by our justice system," said the NUM spokesperson Lesiba Seshoka.
"Cosatu thinks the sentence is entirely inappropriate and just reinforces the view that if you have money you will not be held to account," spokesperson Patrick Craven said.
Brown was recently convicted after admitting to misrepresentations he made regarding investments entrusted to him.
Western Cape High Court judge Anton Veldhuizen handed down Brown's sentence on Wednesday.
Veldhuizen said a R75 000 fine, or 18 months imprisonment, was being applied for each of two fraud counts.
Brown was also given an 18-month jail sentence on each count, but these were suspended for four years on condition he not be convicted of fraud again.
Marele said the two counts on which Brown was convicted involved funds intended for the beneficiaries of Mantadia [Matco], subsequently renamed the Living Hands Umbrella Trust.
"It is the FSB's view that today's sentence neither acts as a deterrent, nor does it send the right message to other white collar criminals," she said.
The sentence also did "very little" to inspire confidence in South Africa's judiciary.
The FSB nonetheless respected the court's decision, but hoped the state would appeal for a harsher sentence.
Seshoka said the Mantadia asset trust company was responsible for paying money from the mineworkers' provident fund to the widows and orphans of workers killed in mine accidents.
He said the union was, therefore, angered at the light sentence Brown received.
"Not so long ago, someone who stole R100 was sentenced to 10 years," he said.
Craven said Cosatu was "outraged" that Brown's sentence was not heavier and called on government to review the court's dismissal of other charges against Brown.
The state originally laid 192 charges against Brown.
"We call on government to announce on its intention to appeal by [Monday], failing which we will embark on protest actions at the Western Cape High Court until clear commitments are given for some prosecutorial actions," Craven said.
Sentencing Brown, Veldhuizen said he had not been convicted of stealing millions from pensioners and Fidentia, or of defrauding either party.
Rather, the fraud of which Brown was convicted of was "an extremely diluted version of fraud, which the indictment alleges".
He criticised the state's handling of the case, and asked why the FSB's chief financial officer Dawood Seedat was called to testify only after Brown was convicted.
Seedat testified in aggravation of Brown's sentence. – Sapa