Re. your question 'Does an Australian death cert include a date of marriage?', it depends on which state (all keep their own records), and of course how much the informant knew about the dead person.

This website shows what information should be included, for each state:
www.jaunay.com/bdm.html

The State Records of NSW index to Assisted Immigrants has:

Surname/ Firstname/ Age/ Ship/ Year/ Arriving/ Remarks/ Copy
BURTON Matthew/ 27/ John Davies/ 1855/ Moreton Bay/ and family/ Reels 2137, 2470
BURTON/ Sophia/ 24/ John Davies/ 1855/ Moreton Bay/ and family/ Reels 2137, 2470
BURTON/ John Davies/ - / John Davies/ 1855/ Moreton Bay/ and family; born on voyage (bv)/ Reels 2137, 2470.

(Lucky for John Davies Burton that their ship where he was born wasn't the 'Chowringhee' on the same Reel, entered just before the 'John Davies'!)

Also on board was:
BURTON/ William/ 19/ John Davies/ 1855/ Moreton Bay/ Reels 2137, 2470

This took a bit of finding:
LONG/ George/ 24/ John Davies/ 1855/ Moreton Bay/ or LENG/ Reels 2137, 2470

When they have an "or Leng" under Remarks, one would expect them to include that in the index? Also not found using their wild card "%" until specifically searching that ship and year.

LONG/ Margaret/ 22/ John Davies/ 1855/ Moreton Bay/ or LENG/ Reels 2137, 2470

One would have to look on the reels (or order copies online) to see whether the Burtons and Lengs were on the same voyage of the 'John Davies' in 1855.

Sue