There is a timetable change in Europe June 9th. You didn't mention where you'd like to go as some countries already did provide their summer timetable but some didn't. To plan, use a date before June 9th, there won't be many changes.

You may buy your railpass even just before your trip does start. But you didn't mention if you plan to buy a paper pass (of course this must be sent to Australia and has to be bought earlier) or a mobile pass for your smartphone. You may buy them right now for summer, too. When buying a mobile pass, you need to start your trip within 11 months so you are already in that time frame, too.

It's described here how to set up a mobile pass: https://www.eurail.com/en/eurail-passes/eurail-mobile-pass/getting-started

A paper pass needs to be activated at a major station or when ordering (by choosing a start date).

Most train companies won’t be open for bookings yet. A few publish their schedules 6 months ahead, but 2-3 months is more common. And sometimes less.

This page gives you some guidance https://www.seat61.com/european-train-booking-horizons.htm

As @Hektor says, you can just search for dates next week, and assume the times won’t change much. Search the train times on www.bahn.com - that’s the best place for timetables.

There is also a big Europe-wide timetable change in June, which pushes publication later still.

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Seat 61 is a great website in general. I suggest you look at the Eurail guide there, and the page about seat reservations. There’s also some suggested itineraries, worked out by a proper human! I don’t know where you’re planning to travel, but to pick a random example: London to Vienna.

Many Thanks  Hektor, very helpful both answer and the setup instruction. Much appreciated.

Many Thanks ralderton, very helpful hints, highly appreciated.