Given the birth for Martha BURNS found by Jamjar with same parent names
19572/1869 BURNS Martha parents Bridget BURNS and William WHITE HLLSit seems possible that Martha who was a witness at Mary's wedding may be her sister, so you may wish to purchase the certificate for Martha which should give the date and place of marriage of parents. You probably know that images of Victorian registers can be downloaded immediately fpr $20
https://www.bdm.vic.gov.au/research-and-family-history/search-your-family-historySue also found some other births which may be siblings for Mary.
1856/2866 BURNE Thomas, Parents William and Bridget BURNE, Back Creek
1873/3057 BURNS, Edward, Parents William and Bridget BURNS, Huntly
These three births are in/around Bendigo, Victoria. Mary lists her birth as "Reedy Creek, Victoria" and yes there is a Reedy Creek in central Victoria, near Broadford - however there is a Reedy Creek which is a suburb of Bendigo. IF these births are related then I suspect that Mary's birth may have been the Reedy Creek near Bendigo.
From:
https://localstats.com.au/demographics/vic/northern-victoria/bendigo/reedy-creekReedy Creek (3658) is a suburb of Northern Victoria, Bendigo, Victoria. It is about 63 kms from VIC's capital city of Melbourne. Reedy Creek is in the federal electorate of McEwen.Plenty of
Reedy Creeks in Victoria - others near Chiltern (Reedy Creek, Chiltern-Mt Pilot National Park), near Eldorado, near Lorne.
Judith