Dress "code" in Australia - Opera and Race Tracks

I will be in Australia in March/April 2025 and will take a pair of black pants and decent top to have something that doesn’t look overly casual.

You’ll be fine in whatever; maybe go for “smart casual”.

Here is what it ways on the opera.org/au website:

It’s a whole lot of fun to get dressed up and be glamorous for a night at the opera. Where else can you wear your grandmother’s pearls or break out your wedding suit?

But it’s not mandatory. Look around the theatre and you’ll see women in cocktail dresses, men in chinos, corporates in business suits, families in their Sunday best and tourists in whatever-they-were-wearing-that-day.

There’s no compulsory dress code. Wear what you’ll feel comfortable in. It can get cold in the theatre, so consider an extra layer. And if you're seeing a show on an outdoor stage, bring warm and weather-proof clothing!

Please avoid wearing strong fragrances in the theatre.

https://opera.org.au/features/the-ultimate-guide-to-going-to-the-opera/

I haven’t been to the Opera House for a few years, but last time I was at the Theatre Royal in Sydney there was a guy wearing a Sex Pistols tshirt in front of me. And he didn’t particularly look out of place. My impression is that dress codes for the theatre have become noticeably more relaxed in the past few years. Nice top and pants are fine.

I believe that you may be overthinking this. The Aussies are generally more relaxed than we are about such things.
Per Lola, wear whatever you'd wear to a similar event here in Colorado and you'll be fine.

Just don't come straight from the snorkel boat without changing.

:-)