Hi Darren,
I had a search of the Western Australian BDM index and found no records for William's marriage or death.  You can search the site yourself at:
http://www.justice.wa.gov.au/portal/server.pt/gateway/PTARGS_0_2_323_201_0_43/http;/justicecontent.extranet.justice.wa.gov.au/bdm/default.asp?structureID=96554822&ResourceID=295610339To try spelling variations, type in part of a surname followed immediately by an *.  I did try that myself but still came up without a result.
These were the results for a search of "William Markham" in New South Wales:
(index no., surname, first name, father's first name, mother's first name, where died)
8000/1906,  MARKHAM  WILLIAM,  THOMAS H  HANORAH,  BELLINGEN
15670/1893,  MARKHAM  WILLIAM,  HENRY  ELIZABETH,  WOLLONGONG  
11961/1882,  MARKHAM  WILLIAM,  WILLIAM,  DIED SHOALHAVEN  SHOALHAVEN 
9306/1885,  MARKHAM  WILLIAM G,  WILLIAM  ELEANOR A,  LIVERPOOL  
8734/1945,  MARKHAM  WILLIAM AUGUSTINE,  JAMES  ELLEN,  COWRA  
18670/1924,  MARKHAM  WILLIAM,  WILLIAM  JANE,  GRANVILLE
12171/1925,  MARKHAM  WILLIAM,  CHARLES  WINIFRED,  WOLLONGONG 
15520/1922,  MARKHAM  WILLIAM,  THOMAS  SARAH,  GRANVILLE 
11877/1942,  MARKHAM  WILLIAM,  WILLIAM  MARY,  NOWRA 
14597/1942,  MARKHAM  WILLIAM HENRY,  JAMES  MARGARET,  SYDNEY    
You can do a search of Victoria's historical index yourself at the following site:
https://online.justice.vic.gov.au/servlet/IdxSearch?action=gotoPrePaySecure(suggest putting in a spending limit of AUS$1.00 at a time ... it costs 99c for each page of records found)
I also searched Tasmania to 1899 with no result.  I don't have the indexes for South Australia or Queensland and they are not available online.
Good luck in your search.
Cheers, Brisgirl