According to Inland Revenue Department
The Mainland and Hong Kong have entered into an arrangement for automatic exchange of country-by-country (CbC) reports. The exchange arrangement applies to accounting periods beginning on or after 1 January 2018 (i.e. ended on or after 31 December 2018).
Some Hong Kong entities of reportable groups whose ultimate parent entities are resident for tax purposes in the Mainland have previously notified the Department of their filing obligation under section 58F of the Inland Revenue Ordinance (Cap. 112) in respect of the aforesaid accounting periods, and received notices requiring them to file the CbC returns. Where such entities are now relieved from their filing obligation because of the exchange arrangement, they should notify the Department through the CbC Reporting Portal on or before 31 March 2020. For the notification procedures, please refer to section B.6 of the Guide to Return Filing.