Well, Polish railways website doesn't at all allow purchase of any international tickets, thus no way to book it officially via Polish side, while
German railways (DB) does allow to book and pay for the ticket, but the only delivery option is 'paper ticket mailed to your home'. Additional postal charge in this case is 3,5 EUR (which is roughly 4,4 AUD). It actually not that bad, especially considering the fact that you can purchase discounted 29 EUR ticket (offer called 'Europa-Spezial Polen'), thus total you pay 32,5 EUR (while in case you purchase it at the station you'll pay standard fare 49,1 EUR) and you definitely have plenty of time till May for your ticket to arrive (usually it takes max.2 weeks with non-EU countries)
But if you don't want to proceed with such option, then you shouldn't worry about ticket at all, just buy it already in Poland. May isn't that busy yet and there is quite a few trains per day, so it's not going to be a problem to buy ticket not in advance.
p.s. and again, in case you are ok with bus travel, then there is low-cost bus company operating on route Warsaw-Berlin, it's called
PolskiBus. Tickets are really cheap and easily booked online, but bus travel time is longer.