Using paint and rocks, a woman deserted on an Auckland island for three nights wrote "Help me" on the beach.

Mystery has surrounded the perplexing case of the woman found stranded on the uninhabited Brown's Island after emergency services investigated smoke billowing from the area on Thursday.

Police are still speaking with the 38-year-old, who lit the fires in the hopes of being rescued, and have not revealed how she got there or why she was there.

The woman arrived at the island on Monday.

The Herald on Sunday can today reveal that a dinghy was found floating in the water and that the woman had left a hospital in the early hours of Monday morning.

The Waitemata District Health Board said yesterday they weren't aware of the woman being a former patient of theirs.

The Auckland DHB wasn't able to confirm if she had been in its care, and the Counties Manukau DHB did not respond to a message.

Police were yet to officially link the dinghy to the woman, but it is understood it had no oars and no motor.

The woman told rescuers she "did not know" how she got to the island - but it's not believed she swam there. She said she had not eaten or drunk water for the duration of her stay, and was slightly dehydrated and sunburned.

Police said on Friday they had concerns for the women's mental well-being.

The island, also called Motukorea, is a 5.5km kayak from St Heliers, and about 1.3km from Little Bucklands Beach.

A source said the woman had left a hospital in the early hours of Monday morning, and made her way to the island that day.

"She spent the night outside on Monday, and then on Tuesday broke into the DoC shed. She slept in the shed on Tuesday and Wednesday, and was discovered on Thursday."

The woman used paint from the shed to write, "Help Me" in the sand, and also used rocks to write the same messages, said the source.

She also tried to wave down passing ferries and boats, but to no avail.

It was inside the DoC shed that she also found candles and a match or lighter to ignite them.

"She tried a few times to get the fire going, and on Thursday it started and someone saw the smoke," said the source.

The Police Eagle helicopter responded to reports of smoke, and was surprised to spot the woman sitting on rocks.

"She was just sitting on some rocks, not waving or anything."

When fire crews were taken to the island, the woman was collected.

The source said she was apparently "really, really quiet and didn't say much", but wanted to apologise for the fire, which quickly raced out of control.

"She wanted to say sorry to the fire fighters and was upset the fire had gotten out of control."

Fire Service spokesman Shaun Pilgrim said the fire was completely extinguished on Saturday morning, with 20 personnel still at the scene yesterday.

Dangerous trees were being felled, and crews were expected to leave about 2pm.

Rescuers described seeing the woman looking dishevelled, tired and upset when they arrived, but said they were mainly concentrating on extinguishing the fire.

Nick Mead, of Auckland Sea Kayaks, said the woman would have been passed by ferries on dozens of occasions - with some less than 200 metres from shore.

"It's a very tall story that she's been out there for four days," he said. "As soon as the story was told it sounded like a tall story.

"It's a busy waterway with boats going by all the time."

He said the Pine Harbour ferry and Waiheke car ferry both passed inside 200m from the island meaning it was highly likely she would be seen "if she's standing there waving at the boat skipper every time he goes past".

"Where she lit the fire, no boat can come in. It was probably the worst place to light the fire. What I would do is exactly what she didn't do."

Mead said the reports of dehydration also had him puzzled because the island had two large freshwater ponds.

Auckland Sea Kayaks offers tours to islands across Hauraki Gulf and had the Department of Conservation concession to take tourists to the island.

He had taken a trip to the island to check on the damage and said "it's looking pretty shocking at the moment".

The kukuyu grass which had burned so fiercely had grown well since the last stock were removed from the island as part of a pest eradication scheme, protecting birds including dotterill who nested there.

Before the fire, the grass was waist high and dry underneath. It is now scorched to the earth.

- Herald on Sunday