mandolino

Strictly speaking, only if Australia found out.

In the late 1990s, having lived in the UK on my Australian passport for nearly 20 years, a UK immigration official leafing through my passport asked why I didn't apply for UK citizenship.
"Because I would have to give up my Australian citizenship" I said.
"Who would tell the Australians?" he said "We certainly wouldn't"

A few years later, in 2003, another incident clarified the way things worked at that time.
I had had two Australian passports for a number of years, for business reasons.

I entered Australia, from India I think, and the immigration officials told me my passport was cancelled and confiscated it.
They told me I could still enter because "We can't deny entry to a citizen" and that I could get a new passport the following week and gave me a number to call. (It was a Friday night)
Stunned, and worried because I was meant to be going on to NZ on the Tuesday, I called the number from my hotel that night, nearly 9pm, intending to leave a message venting my frustration and worries.
To my surprise, a supervisor answered.
"Ah, yes, I can see what happened" she said. "When you renewed your second passport, they cancelled this one. Even though there is a note on your file saying 'do not cancel' . I'll just 'un-cancel' it now"
And she did, just like that.
"But" I said, "I've been going in and out of UK, Russia, India etc on this passport since it was cancelled - how does that work?"
She answered, "We only share passport info with two other countries: NZ and USA. If you had tried to enter either of those on this passport you could have been in trouble"

That will certainly have changed since 2003, especially with the Five Eyes agreement, but that's how it was then and I was very surprised .