On advice from the Australian Government Department of Home Affairs who issue visas, the Event Organisers do not provide Letters of Invitation for accepted presenters as part of the Call for Abstracts process. Their advice is that, if you meet the entry requirements for a visa, your program notification of acceptance and registration confirmation letter and paid tax invoice (received once you complete your conference registration including payment) will be sufficient evidence of your attendance at the conference. The event organiser can supply your attendee information as a PDF document on request.
Further information can be found at Department of Home Affairs – https://immi.homeaffairs.gov.au/visas/getting-a-visa/visa-finder/visit.