Only really two possibilities...well in my opinion
.....your mention of Rockhampton may have created a red herring
Prefix B = Brisbane event C = Country event
Birth
1884/C25
McMAHON Agnes Mary
Father John James Mother Mary McMAHON
Marriage
1913/C3065
TUSTAIN Frederick Thomas
McMAHON Agnes Mary Francis
Death
1970/C7449
TUSTAIN Agnes Mary Frances
Father John James Mother Mary McMAHON
South Rockhampton Cemetery
SRC
TUSTAIN Kelsen 10 months M Sunday, 1 February 1920 RC 33X 4595 Gastro enteritis
North Rockhampton Cemetery
TUSTAIN Agnes Mary Frances F 86 Sunday, 29 November 1970 Monday, 30 November 1970 RC C L 56
TUSTAIN Frederick William (ASHES) M 85 Thursday, 22 July 1999 Saturday, 14 August 1999 28 4 36
TUSTAIN Madge F 83 Sunday, 11 August 2002 Wednesday, 14 August 2002 CATHOLIC 28 4 36
ORAn online tree states that the parents of Maria Agnes
McMAHON were married at
Amarac however they did not have a son John James. Mother Ellen
DWYER died in 1884 and Michael remarried in 1900 to Agnes Mary
HOWARD.
Electoral Roll
1903 Capricornia/Calliope
McMAHON Agnes Mary Calliope HD
McMAHON Michael East Stowe GladstoneCarpenter
1905
McMAHON Agnes Mary Targinnie HD
McMAHON Michael Targinnie Carpenter
1908
McMAHON Agnes Mary Glenlyon Street Gladstone HD
McMAHON Michael Glenlyon Street Gladstone Carpenter
1925
McMAHON Agnes Mary Auckland Villa, Tank Street, Brisbane HD
McMAHON Michael Tank Street, Brisbane Miner
Birth from 1st marriage
1880/C4129
McMAHON Maria Agnes
Father Michael McMAHON Mother Ellen DWYER Born 11 Sep 1880
Married 3 Sep 1907 and is shown in index as Agnes Monica
1907/C1579
MARTIN George William
McMAHON Agnes Monica
Australian Electoral Roll
1908
MARTIN Agnes Maria Thomas Street, Ipswich HD
MARTIN George William Thomas Street, Ipswich Moulder
1913
MARTIN Agnes Moria Musgrave Street, Bundanba HD
MARTIN George William Musgrave Street, Bundanba Moulder
1925
MARTIN Agnes Maria Frederick Street, Newtown, Ipswich HD
MARTIN George William Frederick Street, Newtown, Ipswich Ironmoulder
1936, 1937
MARTIN Agnes Maria Nitram, 21 Frederick Street, Newtown, Ipswich HD
MARTIN Esme Edith 21 Frederick Street, East Ipswich Nurse
MARTIN George William Nitram, 21 Frederick Street, Newtown, Ipswich Iron Moulder
1943, 1954
MARTIN George William Nitram, 21 Frederick Street, Newtown, Ipswich Iron Moulder
Deaths
1941/B052778
MARTIN Maria Agnes
Father Michael McMAHON Mother Ellen DWYERDied 16 Jul 1941
1959/B033155
MARTIN George William
3 Aug 1959
Father William Mannering Mother Elizabeth HEWSON
I have probably created more confusion.
All very speculative.
Cando