The Almighty Sometimes is about Anna, an 18 year old who was diagnosed with a range of mood and behavioural disorders when she was 11, and who has been medicated ever since. Now 18 she has decided to stop her medication, partly to rid herself of the labels that have defined her young life, but also to discover her true self. It’s a choice that’s challenging her mother Renee, who has nurtured Anna and fears she will experience anguish all over again.

Kandall Feaver, who lives between London and the NSW southern highlands, wrote the play in response to what she saw as “a fault-line emerging, a generation-specific issue” affecting gen Y (which the 29-year-old is part of).

“We’re the most medicated [generation] in history [in terms of mental health] and [that brings with it] an issue of parenting that previous generations just haven’t had to tackle,” she says. “I was seeing a lot of mental health advocacy plays and films that mine mental illness as a dramatic tool, but very few that were interrogating what was actually happening. A lot of the young people I talked to were quite angry and I wanted a more authentic experience.”

Feaver spent years researching the play, speaking to psychiatrists, psychologists, mental-health advocates and critics. She also read first-year-university psychiatry textbooks to make sure her material was as accurate and authentic as possible.

The play is named in light-hearted acknowledgement of the frequency with which the word “sometimes” appears in diagnostic tools. People are asked: How often do you feel sad/anxious/depressed? Never, always or sometimes?

The Almighty Sometimes earned Feaver the 2015 judges award in the prestigious Bruntwood Prize, Britain’s biggest prize for playwriting. It was first staged at Manchester’s 700-seat Royal Exchange and was a breakaway hit, despite an inauspicious start. “It was a new play about mental health from a new playwright, completely untested, so as you can imagine it didn’t sell that well,” she says, laughing.

Then the reviews arrived. Four out of five stars from the Times, the Guardian and the Telegraph, an astonishing result for a first-time playwright.

“I was bracing myself for anger – ‘how dare you?’ – [and] worried people would think I was adding to the stigma. But in fact it was the opposite; people were really grateful for an accurate representation of how difficult it can be,” she says.

The Almighty Sometimes is playing at Griffin Theatre with a fresh local cast directed by artistic director Lee Lewis. It features A Place to Call Home and Puberty Blues’s Brenna Harding and Talk and The Kitchen Sink’s Hannah Waterman.

Unfortunately the play is more relevant than ever. The number of children being diagnosed with mental health and behavioural conditions is increasing. The Black Dog Institute found in 2016 just under one in four young people aged 15-19 met the criteria for having a probable serious mental illness – rising from 18.7 per cent in 2012 to 22.8 cent in 2016.

“The numbers have risen across the Western world and in Australia, even though the rates of suicide have fallen for adults. They are consistently rising year-on-year for young people. Suicide is now the number one cause of death in young people [15 to 24 years old],” she says.

Despite the serious nature of the topic, The Almighty Sometimes rings with humour. “One of the things I noticed when researching the play is that people in difficult situations often develop extraordinary humour as a coping mechanism, and I wanted to capture that,” she says.

Following its Sydney season it will be staged in London, and there is interest in a US production. Feaver is currently working on a commission for New York’s Manhattan Theatre Club about womb technology, a topic that speaks to her fascination with bioethics. And she is developing a musical for the National Theatre in London.

Delighted as she is with the play’s success, she is gratified on another level, too. “New writing by women rarely gets on the main stage [in the UK and] to have a play with three intelligent, strong-willed women navigating something that’s political and personal is a real thrill.”

The Almighty Sometimes plays Griffin Theatre’s SBW Stables, Kings Cross, until September 8.