I was disappointed. Oddly, I think that was the point.

At first blush, the concept behind Sia’s latest release seems gimmicky – an exercise in the singer playing at not being Sia. At worst, it comes off as an attempt to capitalise on songs others had rejected, releasing them with her original vocals and packaging the offcuts as a studio album.

Such criticisms, of course, risk taking the enterprise too seriously. With her seventh studio album, the first released since 2014’s 1000 Forms of Fear brought her the widespread renown that had thus far eluded her, the now-ubiquitous Australian takes evident pleasure upending her reputation as a balladeer and singer of pick-me-up anthems. 'Reaper', a perhaps unexpected collaboration with Kanye, finds her drawling the slightly affected patois employed to great effect on 'Chandelier' over a softly chugging groove; 'Sweet Design' finds her rapping about her junk, of all things. It’s brief and messy; delightfully so.

Even those tracks that play to type upend her songwriting formula. 'Unstoppable', which recycles 'Titanium' and this album’s 'Bird Set Free', is clichéd karaoke until you untangle the laughably bad lyrics like “I’m a Porsche with no brakes” and “I don’t need batteries to play” from among the raspy vocal acrobatics. 'Footprints', a lyrical reference to the hackneyed Biblical poem, is a platitudinous middle-of-the-road religious song Beyoncé was never going to release, but is mischievously undone by Sweet Design, which follows (also written for Beyoncé) with its lyrics: “Bump, bump, I’mma rub it up on you / My peach, juicy soft and so delicious”. We’re a long way from 'Big Girls Cry'.

All but one of these tracks was written for other artists, including Adele, Rihanna and Shakira. The resulting lack of coherence is not necessarily a problem on a pop album: half of the fun is imagining what the intended artists would have done. The problem comes once you ask why some of them were rejected.

Musical ideas are shamelessly recycled, as when the caterwauling on ‘Bird Set Free’, the album’s opener, picks up exactly where 'Dressed in Black', the closer on 1000 Forms of Fear, left off. Some of the production is dated: the two key changes in 'Broken Glass' don’t channel Beyoncé’s 'Love On Top' so much as Westlife at their most turgid; 'House on Fire' is a potentially powerful chronicle of an abusive relationship sung flatly over nondescript perkiness. There was no need for Shakira to release the chaotic 'Move Your Body' when the “feel my rhythm in your system” vibe was far better articulated on 'Hips Don’t Lie' ten years ago.

Those songs that echo her classic output also pale in comparison. Gone are dark, intelligent metaphorical flushes (“you’re twisted up like a slipknot / tied by a juicehead who just took his t-shot, and I know / there’s a hungry dog tugging at your frayed ends”). Instead, we have awkward similes (“We fall down like dogs playing dead / Well our love’s not worth playing chicken with”) verging on the bizarre. It leaves you wondering what the album’s irony was intended to achieve.

And then the musical chameleon drops all pretence when 'Space Between' reveals the chanteuse of moving ballads like 'Lullaby' that won over her early fans. A natural showcase for her rough, soulful vocals and near-unparalleled ability to write and perform songs that find their home in montages during the season finales of American TV series, the track is her most emotionally direct, the end of the affair she began in 'Fire Meets Gasoline' (“We’re too tired, we let the embers cool”). It’s a powerful reminder of her uncanny knack for penning piercing metaphors, undoing the rest of the album’s mischief.

Frustratingly, it also suggests the seeds of a much sharper musical project. Here’s hoping that’s where she’s heading.