When beginning her sentencing remarks, Justice Sally Fitzgerald turned to Johnson and said she "fully accept that you love your son very much".
"Putting aside recent events, you have always sought to care for him," she said. "The facts are unique and, on any view, it is a tragic case."
In the hours before Johnson tried to kill her son, her partner noticed her holding her hand to her ear and talking as if she was on the phone, the court heard.
She was also muttering to herself and had a delusion that her partner had sexually assaulted her son.
While later leading her son down to the estuary at Weymouth Park, Johnson was seen by some members of the public.
She spoke to one and mentioned an apocalypse and armageddon, the court heard.
After Johnson and her son arrived at the beach, however, they were not seen again until the next day.
Crown prosecutor Mark Williams said Johnson's son, who was entirely dependant on his mother, was led into the water until he could no longer touch the bottom.
Johnson then sang to him and said "we're off to see Jesus", the court heard.
The next morning, at 7am, Johnson's son was seen by a dog walker, he was standing on some rocks about 40m from the shoreline at low tide.
Thankfully he was not seriously harmed, Justice Fitzgerald said, other than having some scratches and abrasions.
The police were called to rescue him, but Johnson had also called police and told them she had drowned her son.
Knowing he could not swim, she said she made him doggy paddle until he was tired before pushing him under the surface.
Johnson said she then tried to drown herself, the court heard.
At trial, however, Johnson said she had no memory of the events.
Justice Fitzgerald said at time of the offending Johnson was suffering from chronic sleep deprivation and had not slept for 48 hours before she tried to kill her son.
Her mental state was "directly causative of the offending", the judge said.
Johnson continues to deny what happened that day and says she cannot remember the events or talking to police the next day.
Johnson's lawyer, Paul Borich QC, said the trial had demonstrated his client was a loving and caring mother, "except for this moment in her life".
Looking after her son and his needs, he said, was a task she could never successfully provide long term.
"She was in a state of complete and total mental collapse," Borich said. "The circumstances were as unique as they were tragic."
Johnson will serve a period of eight months' home detention and over time be able to contact her son again, Justice Fitzgerald ruled.
Where to get mental health help
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youth services: (06) 3555 906
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• The Word
• Depression helpline: 0800 111 757 (available 24/7)
• Rainbow Youth: (09) 376 4155
• CASPER Suicide Prevention
If it is an emergency and you feel like you or someone else is at risk, call 111.
