Normally, the part time consultancy is not barred under any law. But ethically, one should not do consultancy in the same field in which he is performing his duties in the organization in which he is working, or in the line of business of the organization.
Even Government Officers (Even Secretaries), when invited to do some assignment on an invitation, seek permission from the government to attend the same. Without permission, they cannot do any such assignments. I worked with the Government and it is a reality.