If publication date is not available use no date or circa (about.approximately) c. before the year
For sources that do not have a date of publication, substitute ‘n.d.’ (no date) after the name of the author.
Rule (Author n.d.) or Author (n.d.)
(Southey n.d.) or Southey (n.d.)
Southey R (n.d.) The life of Nelson, Blackie, London, England.
Approximate date or estimated date
In text citation
Rule: (Author c. Year)
Rule: (Author Year?)
Australian Government (2021) Style Manual, accessed 24 May 2021, https://www.stylemanual.gov.au/style-rules-and-conventions/referencing-and-attribution/author-date