If the strata manager has been previously authorised to issue a notice to comply, then this is a valid notice. See Strata Schemes Management Act 2015 (NSW) Section 146:
https://legislation.nsw.gov.au/view/html/inforce/current/act-2015-050#sec.146
There is an information sheet about neighbourhood noise from the Environment Protection Autority:
There is no specific mention about noise from people talking. For the things that are covered, noise after 10:00PM is not permitted. These rules come from the Protection of the Environment Operations (Noise Control) Regulation 2017 at:
https://legislation.nsw.gov.au/view/html/inforce/current/sl-2017-0449