As a vegetarian, Higgins promoted the health benefits of not eating meat in a 2005 advertising campaign by People for the Ethical Treatment of Animals (PETA); and has supported their anti-fur stance. She is interested in environmental issues and is involved with the Sierra Club, a grassroots organisation based in California. She has protested against the proposed industrialisation of the Kimberley region of Western Australia and donated the royalties from her 2009 EP More Than This. Since early 2007, Higgins has tried to make her tours carbon neutral, she purchases green energy to power venues, uses hybrid cars where possible and purchases carbon offsets.

From 2004 to 2007, Higgins' sexual orientation was the subject of media speculation based partly on interpretations of her lyrics and her interviews.[A] In an October 2007 interview with Australian lesbian magazine Cherrie, she was asked if she fell under the moniker of "not-so-straight" girls. She replied "Um, yeah, definitely. ... I think sexuality is a fluid thing and it's becoming increasingly more acceptable to admit that you're that way." In November her MySpace page reported, "I’ve been in relationships with both men and women so I guess I fall most easily under the category 'Bisexual'". She went on to say that she wanted future interviews to focus on her music rather than her sexuality. In a March 2008 interview with AfterEllen.com, Higgins said that her song "Secret" was written about an ex-girlfriend who was not comfortable, at first, about going public with their relationship; "I was so head over heels in love with her I kind of wanted to shout it out to the world, so it was just a song about keeping something under the covers ... keeping it away locked in a little room."

Higgins has been a patron of Australian mental health charity One in Five since 2003. She described her younger self as "a bit of a depressed child" and "introverted", and that she had "experienced various degrees of depression". Prescribed antidepressant medication while in high school, she learned to channel low moods into song writing, calling music her "emotional outlet". In a 2006 interview she said that her songs were "coming from more of a happier place". While recording her second album she discovered a passion for rock climbing, as a "meditative pursuit" and that, "It's the first thing I've had—other than music—that I'm passionate about."

Higgins grew up in the 1980s and 1990s listening to artists that her older siblings liked—Nicola played Mariah Carey and Whitney Houston, while David favoured Queen and Kiss. Departing for boarding school at age 13, she was exposed to alternative artists like Nirvana and Courtney Love and started teaching herself guitar and writing her own music. She also began singing with David's jazz group on weekends. As an adult she prefers Nina Simone and Ray Charles to "poppy dance music". She has cited Patty Griffin, Ron Sexsmith, Rufus Wainwright, Paul Kelly and Sarah McLachlan as influences. Higgins started writing for her yet-to-be-released third album in 2009; the material is influenced by ambient music from Low, Jon Hopkins, Icelandic band Sigur Rós and Estonian classical composer Arvo Pärt.

Higgins' song writing grew out of a desire to express her emotions when she was at school and her lyrics describe her feelings about her own life and relationships. The piano was the first instrument she learned to play, and she continues to use it as well as digital pianos including a Roland RD-300SX, RD-700 and KR-15. She also uses guitars extensively in her music particularly when touring, due to their portable nature and favours the Australian brand, Maton. On occasion she plays xylophone and melodica during performances.

Higgins has won seven ARIA Music Awards from nineteen nominations. She has won two APRA Awards for songwriting and an MTV Australia Video Music Award.