She did not seek senior specialist advice even though Phoebe was screaming in pain and bleeding.
The nurse also ignored 'red flag' warning signs and told doctors the insertion was successful, before feeding Phoebe via the tube and sending her home.
Milk fed into the tube leaked into the hole created between her vital organs and the infection poisoned Phoebe's blood.
Phoebe suffered three cardiac arrests resulting in brain damage and ultimately her death.
Nash was cleared of manslaughter in July last year but admitted contributing to Phoebe's death at the disciplinary hearing.
Lewis MacDonald, for the NMC said: "On 24 August her [Phoebe] parents took her to the A&E department of Weston General Hospital for a feeding tube change.
"It is the NMC's case is that the way Nash did that change resulted in the tube not being properly inserted into the stomach.
"There were a number of warning signs this was the case but they were not picked up."
"The tube was inserted into the abdominal cavity instead of the stomach causing inflammation and acute infection."
Phoebe had cystinosis, a rare genetic condition and relied on the tube to give her sustenance.
She was diagnosed with the rare genetic condition as a baby and had to have the tube changed every three months this was often done by her parents 'without issue'.
Following a procedure at Bristol Children's Hospital the tube needed to be changed in what should have been a simple task.
But there was no bed space at the hospital so Phoebe was sent to Weston General.
But no-one on shift at the hospital was qualified to deal with the tube so Nash, who was a community nurse employed by a Nutricia Ltd was called to the hospital to help.
MacDonald said that the tube should never have been reinserted after a two-hour window.
But Nash arrived at the hospital well over the two hour time limit as she was stuck in Bank Holiday traffic, the hearing was told.
MacDonald said: After one hour it is difficult, after two it is impossible.
Despite this she attempted to insert the tube and this caused bleeding, this was a red flag warning.
She used the tube with some force, the NMC says.
She flushed it with water and the patient screamed out in pain, another red flag.
But even after further warning signs she confirmed to a doctor she had inserted the tube successfully and fed Phoebe milk, it was said.
After the feed there the tube leaked, another warning sign, but again Nash did not seek specialist help from a doctor, the hearing was told.
Mr MacDonald said: 'She should have called on senior advice to further investigate with a CT scan or surgical intervention.'
Instead Phoebe was discharged and taken home.
The next morning she was rushed back to the hospital in an ambulance and it was found the tube had been inserted wrong and the tip was not on the stomach.
MacDonald said: "A significant amount of milk was found in the peritoneal cavity.
"This in turn contributed to her death that day. The NMC says a number of failures contributed to this regrettable outcome.
"She should not have used force in her attempts to so this.
"It should have stopped long before it did."
Mr MacDonald said the hearing would hear evidence that Nash's actions amounted to 'grossly negligent care' and fell 'far below the standards expected of a reasonable and responsible registered nurse'.
Nash, who was present at the hearing in Stratford, east London, today admitted to contributing to Phoebe's death.
She admitted she did not identify that the gastrostomy tube was not in the stomach and accepted this contributed to the death.
The nurse also admitted to not keeping contemporaneous records or doing a risk assessment.
But she denies she inappropriately tried to replace the tube or that she 'forced' the tube into Phoebe.
Nash also denies that she did not seek 'urgent advice' on discovering Phoebe was in pain and bleeding.
She also denies she ignored the 'red flag' signs: Pain on feeding, signs of distress/physiological instability, prolonged or severe pain post procedure, fresh bleeding and external leakage of gastric contents.
The hearing, which is expected to last eight days, continues.
