She's currently singing Brünnhilde in the Royal Opera House's ongoing Ring cycle, where her radiant voice and stage presence were shown off to fabulous advantage in the second opera, Die Walküre, earlier this year. I caught up with her just before last Sunday's first night of the third opera, Siegfried, in which the brawny eponymous hero awakens Brünnhilde at last from her enchanted sleep.
"This is where Brünnhilde has to let go of her godliness as a Valkyrie," Gasteen says. "It's a real struggle. She's fighting to face the fear of becoming human and a woman, the fear of the unknown, the fear of being in love - because love equals vulnerability. And at the end she's wild; it's a reckless outpouring."
She's thoroughly enjoying working with the conductor, Tony Pappano, the producer, Keith Warner - "He has a very clear vision, no mucking around changing things at the last minute" - and her Siegfried, the Cornish tenor John Treleaven.
"He's a darling! We're having a wonderful time. It's fantastic when you have a partner with whom you're really comfortable and who'll act with you, connect with you and play off you."
Gasteen's path from Brisbane to Brünnhilde has nevertheless been fraught with challenges, and although her forthright, no-nonsense personality matches her formidable voice, one senses in her a vulnerability that no doubt feeds valuably into her interpretations.
"I think you have to have had a certain amount of life experience to do justice to the big Wagner roles," she says. "You need to have suffered - and anyone who's lived for any length of time ends up suffering somewhere along the way."
Her bent for Wagner was obvious from the outset - too early, in her view. The demands that Wagner places on his singers are so extreme that it's essential for performers to be mature, strong and able to summon the necessary stamina.
"At one point I stopped singing German repertoire altogether for eight years," Gasteen explains, "because I was being offered innumerable Brünnhildes before I was ready for it. I wish I'd had the chance to do more Italian opera before moving on to Wagner, but you're limited not only by the scope of your own voice - you're limited more by people's perceptions of you. If they can't visualise you in certain roles, they're not going to ask you. But I've given up hankering after things that aren't going to come my way."
How did it all begin? "Boredom! I was in a job that I hated, working in the family dry-cleaning business. I began to think about what else I could do. To get into uni I'd have had to repeat Grade 12 [Australia's equivalent of A-levels] and get the right grades, so I thought maybe I'd give singing a go. There was a preparatory course at Brisbane Conservatory where you could get in on talent alone. I thought at first I'd do jazz or nightclub singing. But I started being taught in the classical Italian style, my teacher discovered that I was a soprano, not a mezzo, and then I got bitten by the bug."
In 1991, Gasteen's voice blew away the jury and a nationwide TV audience at the Cardiff Singer of the World competition. Not that coming to the UK helped much when Gasteen accepted some Verdi roles in Wales and Scotland. "Like most Australians, I was badly affected by the weather. I was sick almost the whole time." Moreover, she was living in a house that, unknown to her, was full of concrete dust, which made her ill and forced her to cancel performances. "That was damaging - it set me back years. But you just pick yourself, dust yourself down, wipe away the tears and get on with it. It's all a long time ago now. And while I wasn't so busy, I was able to have my two children, so that's wonderful too."
With her husband, a lawyer, she's finally moving to the UK, the solution to the long absences that can make an international career such a stress on relationships. "The music's wonderful, the career's great, but it's not good for family life," she declares.
"As a young singer, I'd hear people talk about the sacrifices they'd made for their careers. I always thought that if I got to that stage, I'd stop. But when I did get to that stage, it was a shock to find I couldn't stop.
"It's not only the performing, the need and the drive. It's the challenge. Nothing challenges you as much as this. Once you've had the constant challenge of performing at a high level, nothing else can really satisfy you."
'Siegfried', Royal Opera House, London WC2 (020 7304 4000)
Four Wagnerian greats
Born in Oslo in 1895, she became an idol to Wagnerians the world over. Her New York Metropolitan Opera debut as Sieglinde in Die Walküre in 1935 was broadcast nationwide and caused a sensation. Her purity of tone and the creamy, silky nature of her voice made her reputation.
A powerful dramatic soprano, born in Stockholm in 1918, who sang Brünnhilde many times at Bayreuth during the 1950s and 1960s. She shot to fame overnight in 1941 when she stood in for Lotte Lehmann at the Met, in New York, singing Sieglinde in Die Walküre aged only 23. Six days later, she substituted for the ailing Brünnhilde. Her autobiography, Fifty-Five Years In Five Acts, is fascinating on her high-profile falling-outs with conductor Herbert von Karajan and Rudolf Bing, director of the Met.
The incandescent Swedish soprano, viewed as the successor to Flagstad. Her interpretation is immortalised in the 1965 recording conducted by Georg Solti. In a documentary about the recording, The Golden Ring, the cameras recorded a joke in which, when Brünnhilde calls for her horse, a four-legged friend comes clopping into the studio. It is said that Rudolf Bing, asked whether Nilsson was "difficult", said: "Not at all. You put enough money in and a glorious voice comes out."
Slender, golden-haired and blessed with a searing voice, the Welsh soprano dominated the Bayreuth Festival's soprano roles during the 1960s and 1970s. Like Varnay, she had her first big break as a last-minute substitute, standing in for Leontyne Price in 1964. Her all-giving Brünnhilde was one of her supreme achievements, especially at the Bayreuth centennial production conducted by Pierre Boulez in 1976.