"A patient with abdominal pain waiting to be seen could just be having gas pains or appendicitis," she said.
"A patient with chest pain waiting to be seen could be just having heartburn or a life-threatening heart attack.
"An overwhelmed and rushed junior doctor may miss a potentially life-threatening, time-sensitive diagnosis. It's difficult to think critically when one is constantly bombarded, which is sometimes what happens overnight," Na said.
At the heart of the issue is the overnight roster which has been described in a letter sent to the board and signed by 26 doctors as "unacceptably unsafe".
And it had led to a situation where senior doctors were taking it upon themselves to fill gaps, largely uncompensated, Na said.
"Moreover, the SMOs [senior medical officers] stay routinely late (about two-three hours, at least two times per week) because we do not feel comfortable leaving RMOs [resident medical officers also known as junior doctors] overnight with a waiting room full of patients to be seen.
"We have been largely uncompensated for such effort and told that we stay late 'by choice'. Such persistent extenuation has taken a toll on our staff. It comes at the cost to our own wellbeing and is unsustainable. Hence, the high turnover and a constant revolving door of foreign doctors."
She said herself and other doctors had requested more staff but they were "yet to receive a serious response from management".
Na would not go so far as to say patients had died because of inadequate staffing.
"The patients who have had bad outcomes had diagnoses with high mortality rates -dissection, head bleeds," she said.
"And though timely intervention would have improved their chances of survival, it is unfair to say "they died because ED wasn't staffed properly".
"I am aware of patients with life-threatening diagnoses who had delays in treatment due to insufficient staffing."
Whanganui DHB's chief medical officer Dr Frank Rawlinson said Na was correct there would have been instances of a junior doctor by themself in the emergency department.
But he said there were mechanisms in place to protect them.
"RMOs in fact at night time are on site by themselves.
"Of course we have our team of very experienced nursing staff who are responsible for the wider hospital that are actually there as the support infrastructure as well as the SMOs that are on call.
"An individual may be required to function on their own for a short period of time if they deal with an acute situation. The ED SMO have to answer the phone immediately - they have to be here within 10 minutes, it's part of their contractual obligations."
The head of the hospital's emergency department, Michael Caruso, said rosters had now changed to allow for there to be at least two junior doctors on at all times.
"We currently have a total of three RMOs on every night ... two are dedicated to the ED itself. We have a third person who's cross cover based in the ED who also will cross cover the wards on an as needed basis."
The DHB said that change had made a "significant difference".
It said it increased the number of RMOs on November 26 but it had for the three months prior been using a third junior doctor overnight at times.
Caruso did not agree senior doctors went uncompensated for extra time they spent helping junior doctors in the emergency department.
He said RMOs and SMOs were compensated in two different ways - either for coming in during after hours or staying late.
