Hi Janet!
I wasn't overburdened with great finds today, but I did get a few bits for you:
From Assisted immigration book 1848-1884Ship: Cairngorm, arr Brisbane 2 Jul 1863
IMM/112 p:205 film:Z1957
McGavin Matthew age 55
Grace age 54
Eliza age 16
Ship Cairngorm from Liverpool via Greenock
Matthew said he was a labourer
(I thought this was not your family, but see Will below)
From Immigration cardsShip details:
“Helenslee” sailed from Glasgow to Brisbane
all on board were “Scotch”
William Brown was commander/master
Arr 9 Aug 1862
On this list are just names – top of page is torn off
Robert McGavin
M?
Janet S
Mary R
Robert
John AS
Matthew
Jane
William
Margaret
Passenger list: (awful smudge through page – some words are unreadable but I think we can make an educated guess as to the names)
Cabin:
Robt McGavin 53 (age hard to read)
? 50 female
Janet S 26 female
? 24 female
? ?
John AS 18
Matthew 7
Jane 16
William 13
Margt 8
I could not find any Thomson or Thompson immigrations that fit your family names – Adam is a distinctive name and I could not find him. Also checked David and tried to match up with the more common names like William, Jane, Robert etc (there were quite a few of them

). Perhaps they went elsewhere in Australia or maybe America? Or perhaps only some members of the family with common names came to Australia? There are dozens of Williams for example

.
As Karen found Margaret’s date of death, I looked for her will, which I found. Karen couldn’t find a death for Robert – I double checked but found nothing either. Have narrowed his death down to between 9 Aug 1862 (date of arrival in Australia) and 23 May 1878 (date Margaret wrote her will – she was a widow then).
I looked for any McGavin wills to 1900 in the shiny new “all wills in all Queensland” book. There were only three:
McGavin Margaret (see below)
McGavin James (I looked twice on this film but, frustratingly, was unable to locate this will – if you’d like I can try again next time I go to the Archives)
McGavin Matthew (see below)
Margaret McGavinOf Rockhampton
Will dated 1878
died 28 July 1886
late of the Rockhampton Agricultural Reserve.
Executors are sons John Adam Skelly McGavin and James McGovern, both graziers, both of the Agricultural Reserve in Rockhampton.
£150 to daughter Margaret McGavin “for her use alone free from any debts or control by a husband”. If Margaret marries her mother states that she wants this money “enjoyed and disposed” of by Margaret as “her sole and separate property, free from marital control or influence”. (Seems like there might be a story behind that

).
She wants to exclude son Robert from all participation in real and person estate (another story

)
Unfortunately there is no death certificate included with the will. This may just be a co-incidence, but the second witness to this will is a Walter Thomson (clerk). Wonder if there is any connection to your Thomson family.
Matthew McGavinOf Brisbane
Died 16 Dec 1874, North Shore Sydney
Was a Minister of the Presbyterian Church
Wife Grace, he left all household goods, furniture and effects and all real and personal estate.
He left Margaret G Bell (his wife’s niece) – bedroom furniture, piano and napery
He left Jane, daughter of his wife & wife of Daniel Clark of Brisbane) all silverware
He left Margaret, daur of wife & wife of James Matthew Banks(?) (I think – can’t read my own writing) of Sydney – mantle clock, and silver spoons marked with his wife’s name.
The rest of household goods, furniture and effects to be divided equally between his children: Robert, James, Matthew and Elizabeth
...more to follow ....