Ross Taylor was in the care of mental health services in Dunedin for a year before his death, both as an inpatient and in the community.
He initially presented with symptoms of psychosis, and his mother said the family was not told by mental health services he was suicidal.
"My son became unwell aged 19 - it was the first time we noticed he was unwell."
The family found out later he was hearing voices instructing him to kill himself.
He had been out of acute inpatient services for just a month when he died.
Taylor started the Life Matters Suicide Prevention Trust in 2014, which has helped run events such as Hope Walks and organised bereavement groups.
She said she felt she had few other options following her son's death.
"It left me with no choice. I'm actually a really private person, this is really hard for me to do."
People were commenting and sharing their stories of mental health issues and suicide as well as signing the petition.
"It's remarkable when they start sharing their stories," Taylor said.
"My personal story is just an echo of other people's.
"When I hear that nothing has changed since my son's death, it breaks my heart."
A report by the mental health commissioner revealed Ross had received "sub-optimal" care.
His anti-psychotic medication was reduced just 12 days after his release from a psychiatric hospital as staff decided he was low-risk and not suicidal.
Mental Health Commissioner Kevin Allan said in his report the Southern District Health Board and a psychiatrist failed to provide services with "reasonable care and skill".
The petition had an initial target of 25,000 signatures, but Taylor said she was now expecting many more as it was already nearing 22,000.
"On Monday I shared it to the Life Matters page and it just went viral."
Taylor urged people to get help if needed.
"They must reach out to one another and they must reach out to the services for help. And don't go alone, get someone to go with you."
People in need were welcome to contact the Life Matters Trust, she said. Details were available at www.lifematters.org.nz and the petition here.
Another woman who lost a child to suicide also started a petition calling for a mental health inquiry, which had nearly 28,000 signatures.
It was supported by the PSA and YesWeCare.nz and was due to be presented to Parliament on Tuesday.
Where to get help:
If you are worried about your or someone else's mental health, the best place to get help is your GP or local mental health provider. However, if you or someone else is in danger or endangering others, call police immediately on 111.
Or if you need to talk to someone else:
• LIFELINE: 0800 543 354 (available 24/7)
• SUICIDE CRISIS HELPLINE: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• YOUTHLINE: 0800 376 633
• NEED TO TALK? Free call or text 1737 (available 24/7)
• KIDSLINE: 0800 543 754 (available 24/7)
• WHATSUP: 0800 942 8787 (1pm to 11pm)
• DEPRESSION HELPLINE: 0800 111 757