Sydney underworld figure George Alex will be given a day's medical leave prior to resuming his evidence before the unions royal commission on Friday.

Mr Alex told the commission on Wednesday he had helped home-grown terrorists Khaled Sharrouf and Mohamed Elomar with their boxing training and had not hired them as 'muscle'.

'They didn't want to train in front of their community and I had premises at my house at the back and they would train there,' according to his testimony.

Elomar and Sharrouf are believed to have been killed fighting with Islamic State in Iraq.

The commission called Mr Alex as part of investigations into the CFMEU and its dealings with suspected phoenix companies - companies that collapse owing worker entitlements then re-register under a new name.

Mr Alex also rejected an allegation he made regular $2500 payments to CFMEU official Darren Greenfield.

Mr Alex suffers ongoing complications with his left leg resulting from an accident, with the commission told he is on heavy pain medication and may need to have his leg amputated.

AAP