The writer of the anonymous letter that sparked a new probe into the death of Zachary Gravatt is suspected to be a senior doctor.

This is the view of the Auckland District Health Board chief medical officer, Dr Margaret Wilsher, who was the last witness to give evidence at the reopened Gravatt inquest, which ended today.

Following the end of the five-day hearing today, coroner Morag McDowell will consider the evidence before issuing written findings and any recommendations she considers necessary.

"On behalf of the ADHB," Wilsher wrote in one of several statements to McDowell, "my opinion is that the anonymous letter could have been written by a senior medical staff member at ADHB with knowledge of DCCM [the Department of Critical Care Medicine].

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"This is because the contents of the anonymous letter display a depth of knowledge about Zachary's care, internal DCCM systems and DCCM staff.

"It is unlikely that a member of the public or a more junior staff member would have this level of knowledge."

The letter was sent to Gravatt's parents Lance and Jennifer. Authorities were alerted and deputy solicitor-general Virginia Hardy ordered the reopening of the inquest. The findings of the first coroner to investigate the case, Brandt Shortland, were issued in 2011.

Zachary died of meningococcal septicaemia at Auckland City Hospital on July 8, 2009.

The DHB asked the coroners' office to order a forensic examination of the letter. Several fingerprints were found, but remain unidentified.

Wilsher detailed allegations from the letter and made reasoned rejections of each.

The allegations she discussed and dismissed were that there had been a cover-up of the true events relating to Zachary's care; that disciplinary action had been taken against staff involved in his care; that an expert who reviewed the case was an inappropriate choice; and that a doctor who received a series of hospital phone calls about Zachary delayed going to the hospital.

Earlier evidence to the hearing indicated that the doctor, whose name is suppressed, took around 45 minutes to get from the Ponsonby restaurant, where he had been eating dinner, to Zachary's bedside. He arrived at around 7pm, 15 minutes before Zachary died.

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The doctor said he was held up by heavy traffic and got to the hospital as quickly as he could.

Wilsher said that despite the doctor being off site when told about Zachary, the doctor had "directed appropriate treatment" which was provided by another doctor.

"There was no delay in Zachary receiving appropriate treatment because [the off-site doctor] was not present when he, Zachary, arrived in DCCM."

Wilsher said that since Zachary's death in 2009, the DHB has substantially improved the way it recognises and manages patients with acute physiological deterioration.

"In the area of sepsis, there has been increased recognition of the need to identify sepsis early and treat aggressively with blood pressure support and antibiotics early.

"Most patients with sepsis [a massive immune response to infection] will now be administered antibiotics within the first hour of diagnosis and have fluids and any other treatment started early. The implementation of the national early warning score ... and other guidelines, along with the use of blood lactate measurement, has enabled early detection of sepsis."