i am still looking for an answer to margaret lucy cook who married ernest albert henderson, then for some reason either married another called roles. 
I can't see where anyone has suggested you purchase 
 certificates, of death,marriage or birth.  I did suggest you may like to purchase information from SAGHS which would possibly have been a cheaper option than visiting and paying to use their library.  It is not a free service.
The information you seek has already found and has been posted on this and the linked thread.  I will detail it again as you don't appear to have read it thoroughly.   Margaret Lucy COOK either had  two marriages or one marriage and a relationship. 
Marriage in South Australia
HENDERSON Ernest Albert 23 years  Status single  Father Arthur HENDERSON
COOK Margaret Lucy 18 years  Status Single  Father Frederick William COOK
9 May 1931
At St Laurences Priory North Adelaide  Ade S 327/406
Marriages in New South Wales
17890/1942
ROLES Ernest Sydney
CRAWFORD Bessie Noele
District Wagga Wagga
2nd marriage for his wife
15019/1945     
HILL Donald Maxwell       
ROLES Bessie Noele     
District Goulburn 
Birth date for 
Ernest Sydney ROLESBorn 14 Mar 1910  Sydney    Source 
http://www.ww2roll.gov.au/Australian Electoral Rolls
NSW 1936 Commonweath Macqaurie  State Yass
ROLES Ernest Sydney  Lorraine, Taralga    Farmhand
NOK  Margaret Lucy 
ROLES   Source 
http://www.ww2roll.gov.au/This indicates to me that between Ernest Sydney ROLES first marriage in 1942 and his date of discharge 17 Dec 1945 [his 1st wife's re-married in 1945], he either married or formed a relationship with Margaret Lucy COOK. There is no marriage record in NSW to 1958 and available marriage records for South Australia only extend to 1937.
If the NOK alters during the period of the service record it appears the the last NOK is entered on the WW2 Roll.
Deaths
ROLES Earnest Sydney
21 Feb 1966  55 years  Status Married  No relative noted
Residence Kilburn  Death Place Glenelg   Ade 6A/1250
Death and In Memoriam NoticesHENDERSON Ernest Albert
Advertiser 1982
ROLES formerly HENDERSON nee COOK    Margaret Lucy [Maudie, Maudy]
Advertiser 1989
http://www.saghs.org.au/graves.htmCemetery IndexROLES Ernest Sydney (Dick)
Source date 1966
ROLES Margaret Lucy
Source date 1989
What other information are you seeking?Cando