When are you planning to travel?
Probably a pass won’t be your best option, as advance tickets will be way cheaper than a pass + reservation cost.
Should you choose to go entirely by train from Italy to the Netherlands a pass already makes more sense. (Rome-Florence-Venice-(maybe add something Switzerland for good alpine views) - Paris - Amsterdam - (Fly back to Australia from Schiphol) will make better sense. Count the amount of days you’ll be taking longer train rides and choose a pass accordingly. Probably will be cheaper than seperate tickets and squeezing a flight in between it.
Get them at the websites of the rail companies or a website like https://www.raileurope.com/
(Italy: Trenitalia, Thalys (soon Eurostar red) for fast trains Amsterdam-Brussels-Paris)