A nurse can't explain the absence of a medical reading which she claims showed normal blood sugar levels in a critically ill woman, an inquest has heard.

Heather Bills, 64, died at Middlemore Hospital on January 2, 2013, six weeks after she suffered burns to 35 per cent of her body in an explosive house fire.

It was later discovered that she suffered a suspicious and massive hypoglycaemia-related cerebral injury, due to a high volume of insulin being "introduced to her body from the outside".

Police suspected three hospital staff members of potentially injecting Bills with the fatal dose.

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The case remains open.

A nurse, who has interim name suppression, was working during a 7pm to 7am shift on Boxing Day, 2012.

Late that day Bills' health began to deteriorate rapidly from a then-unknown cause.

The nurse has spent today giving evidence at a coroner's inquest in the Auckland District Court to determine the source of the insulin.

However, the evidence directly contradicts the testimony of health care assistant Sharon Connors, who was Bills' watch from 7pm to 11pm on Boxing Day, 2012.

The pair are at odds as to the alarms sounding from Bills' monitoring machines and the care given to her.

Connors "seemed oblivious" to the situation, the nurse said.

The nurse later entered Bills' room just after 11pm, after heath care assistant Nirmala Salim raised the alarm, concerned over the grandma's rising heart rate.

Bills' heart rate had reached 145 beats per minute.

Her condition became critical after midnight as emergency doctors were eventually called to diagnose the decline.

The inquest heard the nurse conducted a blood sugar test with a glucometer, without the direction of a doctor.

The nurse told the court a test was performed at about 1.40am because Bills "seemed like she had a low blood sugar" based on her symptoms.

However, according to the nurse, the test came back normal but the nurse couldn't specifically recall notifying the medical team.

Dr Lit Son Yoong previously told the inquest that he recalled asking for Bills' blood sugar levels and was told it was "fine".

"I recall a staff nurse said that blood sugar was 5 or 6, one of those two numbers ... We therefore concluded that she was not suffering a hypoglycaemic episode or a hypotensive," he said.

Blood tests were also conducted by the team at about the same time.

"It was just a nursing assessment that I was doing. It wasn't something that anybody needs to be alerted [to]," the nurse said when cross-examined by Bills' daughter Michelle Maher over the glucometer test.

"It perplexes me," Maher said, continuing to question the nurse about the glucometer reading of 6.4 mmol/L, well within the normal range of four to 11.

Michelle Maher (right), Heather Bills' daughter, says her family deserves to know what happened to her mum. Photo / Michael Craig
Michelle Maher (right), Heather Bills' daughter, says her family deserves to know what happened to her mum. Photo / Michael Craig

However, when Bills' blood tests came back it showed a clear discrepancy to the nurse's bedside test and indicated the blood sugar levels were critically low and the reason for the sharp drop in health.

The nurse said they only learned of the inconsistency between the tests "way down the track".

But, the court has heard the nurse was informed just hours later.

"You were made aware of that on the same shift, on that same night," Maher said to the nurse.

"I cannot explain what - what I'm trying to say is I cannot recall having this conversation," the nurse replied.

Maher then asked the nurse what her immediate reaction was to low blood sugar being found to be the cause of decline.

"What did you do?" Maher asked,

"You're asking me how did I feel?" the nurse replied.

"No, what was your response?" Maher said.

"I would've been surprised to hear that," the nurse said.

An emotional Maher said the glucometer reading was the "one thing that could prove that [the nurse] acted appropriately" and that notifying doctors "could have saved her [mum's] life".

However, there is no material record of the nurse's glucometer reading.

Despite the glucometers being able to show the results of the more than 400 most recent readings, none displayed the nurse's 6.4 reading at 1.40am, police and DHB investigations found.

The court heard that the blood glucose reading may have been an error.

"Could be several reasons," the nurse said, adding the error may have been because Bills' hand or finger was not cleaned properly.

The nurse said an accurate reading could've saved Bills' life.

"I'm sorry," the nurse said.

Maher asked the nurse if she was aware of the use of insulin around the world to assist is "mercy killings".

"No," replied the nurse.

"Did you inject my mum?" Maher continued.

She was given a warning by chief coroner Judge Deborah Marshall - and the line of questioning was objected to by the nurse's counsel, Karen Rose.

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