Ms Cartwright said her mother's head injury required seven stitches and the anaesthetic left her confused.
"They said they were going to send Mum back to the rest home because ... familiar surroundings would be good for her because she was quite dazed and confused.
"I left her about 5pm or 5.30pm and so I thought it would be in a couple of hours at least, but when I got told it was like 1am or 1.30am I was gobsmacked, it's just not on.
"She had just turned 90 and to let her out at 1am or 1.30am, I want to know what their policies are. I can't fault the care she received, the nurses were great, very attentive and lovely. But it was bitterly cold that night."
Forrest Hill staff also reported that when she returned, Mrs Cartwright had an IV attachment in her arm.
Ms Cartwright and Mrs Duncan have laid a formal complaint with the hospital.
"I don't think any patient who has suffered a stressful incident, serious enough to be admitted to the ward, should be sent out in the early hours of the morning and certainly not someone of my aunt's age and condition," said Mrs Duncan.
"I was angry and outraged a 90- year-old who can't walk and whose hands are crippled with arthritis should be treated in this manner.
"I want to know what their policies around this are. I want people to start thinking about the elderly person that has done a lot for New Zealand instead of stereotyping them with 'they're old, get them out of here'."
Waitemata DHB emergency department clinical director Dr Willem Landman apologised and said an investigation into why the IV attachment was left in place had started.
Mrs Cartwright had been cleared to return to her rest home at 6pm, but "due to an excessive delay in securing an ambulance" she was not able to leave the hospital until 1.30am.
"Staff do not send patients home, especially at night, unless patients will be safe and well cared for," said Dr Landman. "In this instance, Mrs Cartwright's residence is a hospital-level care facility.
"An investigation into why Mrs Cartwright's IV Luer was left in is underway and we will be reporting back to Mrs Cartwright's family as soon as possible."
The rest home's acting manager, Kate McGregor, said the IV attachment failing caused "shock and horror" among staff.
"That's not something we would expect at all. It was very much a shock and horror thing. We don't expect anyone to come back to a facility with Luers or any equipment in them."
It was fortunate the home had enough night staff to admit Mrs Cartwright, she said.
Unhappy homecoming
* North Shore sent the nonagenarian home from hospital at 1.30am.
* An IV attachment was still in her arm.
* The late hour was because no ambulances were available earlier.
* An investigation is being made into why the IV attachment was left in place.
