Carterton 13-year-old Grace Yeats says "thank you" to her supporters, and helps her mum Tracy recite the alphabet from her bed in Starship children's hospital on Saturday.

A Wairarapa teen silenced by a rare illness has woken after emergency surgery with a mysterious bonus: for the first time in nearly four years, she can talk.

But even though early signs are that a medicinal cannabis treatment may have contributed to the breakthrough, Associate Health Minister Peter Dunne says there is nothing he can do to subsidise its hefty pricetag of more than $1000 a month.

Carterton 13-year-old Grace Yeats has been unable to move or speak since she was hit by severe basal ganglia necrosis, a rare brain illness, just after her 10th birthday, leaving her trapped, but cognitively unimpaired.

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But on Saturday she began speaking after waking up from surgery in Starship children's hospital in Auckland.

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"Grace is talking! It's amazing, everyone is so thrilled," mum Tracy Yeats said on Monday.

TRACY YEATS/ SUPPLIED Carterton 13-year-old Grace Yeats smiles from her bed in Starship children's hospital on Saturday, after waking up mysteriously able to say some words for the first time in nearly four years.

Grace's words so far include yes, no, please and thank you – none of which she has been able to say since her illness in 2012.

"There's also a lot of 'I love you Mum, thanks Mum' ... it's really nice," an emotional Tracy said.

The breakthrough was revealed when a nurse asked Grace if she was helping her check a tube by adjusting her T-shirt, and Grace matter-of-factly replied: "Yes."

SUPPLIED Grace has had tough times in the last four years, but some highlights too - such as meeting her beloved Taylor Swift in Auckland.

"She's pretty stoked with herself," Tracy said.

So far, Grace has been too excited to speak clearly when excited hospital staff fill her room, or for smartphone videos.

But she did manage a quick video message to her many supporters: an indistinct but recognisable "thank you".

SUPPLIED Grace is making huge progress but there's a long way to go, her mother warns. She is pictured on one of the many tough, painful days she has endured in the last four years.

Tracy said Grace's neurologist and speech therapist were delighted but dumbfounded by the breakthrough.

So far, ideas for the cause of the change included extra relaxation caused by morphine following the surgery, or a cumulative effect of the medicinal cannabis treatment Sativex​ Grace had been using since late last year, Tracy said.

Regardless, she considered the development a miracle. "Grace prays every night for her voice to come back, and she has always believed it would."

Grace's dad Stephen said the news was all the more amazing because Grace was flown to Starship last week for emergency surgery to replace a broken-down pump permanently installed in her abdomen, which releases sedative for her pain and spasms.

"That was a major step backwards, so it's really mysterious."

Tracy said that, despite the progress, Grace still had a long, costly road ahead, with the Sativex spray alone costing $1100 a month.

"It's early days. It's not like she's talking normally, it's going to be a process. But it's a huge step."

Grace's Masterton paediatrician, Andreas Leinfellner, said he was excited at the news.

"I don't have a medical explanation, which I guess makes it a little miracle."

The possible role of Sativex was one of the things he wanted to discuss with Starship's specialists.

Grace's Starship neurologist had to apply for approval from Dunne for her to be allowed to take Sativex, which is not publicly funded.

Dunne said he was "pleased" at Grace's improvement but could not say whether it should influence government drug-buying agency Pharmac's thinking on funding it.

"While it's good news in the case of this particular patient ... I can't draw any conclusions beyond that.

"It's a decision ministers don't make, about the clinical availability of medicines."

Pharmac director of operations Sarah Fitt​ said it considered funding Sativex last year, but decided against it. That did not stop doctors prescribing it, with ministerial approval.

"Pharmac routinely seeks advice where new evidence becomes available and can reconsider an application at any time."

Grace's psychologist, Kirsty Ross, said she had seen noticeable improvements in her body control and movements recently, "and more vocalisations than ever before".

"I am not a medical doctor and so I feel unable to comment about what has led to this new and fantastic development, but I can say I have seen marked progress which correlates with starting Sativex. Young people's brains' neuroplasticity mean that it is hard sometimes to predict outcomes."

Grace and her mother hope to be home within a week.

To find out more, or donate, visit Grace's Givealittle page.