If I register voluntarily though my turnover is less than Rs. 20 lakhs, am I required to pay tax on supplies made post registration

Yes. If you obtain voluntary registration despite the turnover being below Rs. 20 lakhs, you would be treated as a normal taxable person and would need to pay tax on supplies even if they are below the threshold for registration. You will also be entitled to take input tax credit.

(FAQ 19: MSME)