Death documentations whilst immigrating

Hi all,
My late brother-in-law passed away en route to Australia in 1968. He was only 9 months old :’(

Neither of his parents is in a position to answer any questions.

I am trying to find out whatever I can about their trip to Australia, as well as the departure of the husband (& children), back to England approximately 3 years later.

Their application for the Assisted Migrant scheme has recently been digitalised. I thought we had found more information than that on previous searches; however, nothing is showing up anymore.

They travelled via ship from Southampton, disembarking in Sydney. It is noted on one of their application documents that no one has a passport.

Where should I base my search? Are there any key words i could use? The relevant surname isnt giving anything other than what I already have