Ms Williams, 34, competed in Sydney against finalists from Australia, Canada, Israel and Russia for the Elizabeth Connell Prize.
Ms Williams did not win but said: "The standard was extremely high."
Originally from Llanberis, Gwynedd, she currently lives and works in Cardiff.
The prize is worth Â£11,300 ($20,000 Australian dollars) and also includes the chance to audition at the Royal Opera House, London.
She said: "I started singing at the Eisteddfod with my grandmother when I was younger and then I had singing lessons at age 17, which was when I discovered I had an operatic quality.
"It's a big honour to be representing not just Wales, but the UK and Europe.
"It's a huge thing, run by people I really respect.
"Just being recognised in the top five of my voice-kind in the world is amazing."
South African soprano Elizabeth Connell died in 2012 leaving a legacy to create a scholarship in her name for the aspiring dramatic sopranos of the world.
It was her wish for the scholarship to be administered by the Joan Sutherland and Richard Bonynge Foundation and held in Australia.
Aspiring Welsh dramatic soprano Mari Wyn Williams was the only entrant from Europe to make it through to the final five in a world-wide contest to win an operatic scholarship on Saturday.