Sir,
Prior to 1.7.2017, service tax was leviable on audit services. Therefore GST can be shown in the same way how service tax paid for such service was shown. If the bill for audit is raised after introduction of GST then you have to charge GST only.
According to Section 13 (2) of CGST Act, 2017 "the time of supply of services shall be the earliest of the following dates, namely:-
(a) the date of issue of invoice by the supplier, if the invoice is issued within the period prescribed under sub-section (2) of section 31 or the date of receipt of payment, whichever is earlier; or
(b) the date of provision of service, if the invoice is not issued within the period prescribed under sub-section (2) of section 31 or the date of receipt of payment, whichever is earlier; or
(c) the date on which the recipient shows the receipt of services in his books of account, in a case where the provisions of clause (a) or clause (b) do not apply:
According to rule 47 of CGST Rules, 2017 "the invoice referred to in rule 46, in the case of the taxable supply of services, shall be issued within a period of thirty days from the date of the supply of service.