If there is no date use 'n.d.' (for 'no date') in both the in text citation and the reference list.
In text:
In the late 1950s, white Australians became more aware of indigenous living conditions reported in the news (National Museum Australia n.d.).
The civil rights movements started to gain momentum in Australia as "events in the late 1950s brought the sufferings of the few into the living rooms of the many" (National Museum Australia n.d.).
Reference list:
National Museum of Australia (n.d.). The fight for civil rights. NMA website, accessed 18 October 2021.
Regarding reference list order:
If you have several works by the same author, and one of the works has no date but the other works are dated, (n.d.) is treated as the oldest work for ordering your references
If you have multiple citations from the same author which also do not have a date, you will follow the same instructions as you would if there was a date.
Order the citations in your reference list alphabetically by title, and place an 'a', 'b', etc after the date.
Reference list:
National Museum of Australia (n.d.-a). Nelson the Newfoundland's dog collar. NMA website, accessed 18 October 2021.
National Museum of Australia. (n.d.-b). Newcastle bakery cart. NMA website, accessed 18 October 2021.