Flight times from Australia and New Zealand to Prague are twenty hours or more, depending on routes and transfer times. There’s a wide variety of routes, with those touching down in southeast Asia the quickest and cheapest on average. Given the length of the journey involved, you might be better off including a night’s stopover in your itinerary, and indeed some airlines include one in the price of the flight.
The cheapest direct scheduled flights to London are usually to be found on one of the Asian airlines. Average return fares(including taxes) from eastern gateways to London are Aus$1500–2000 in low season, Aus$2000–2500 in high season. Fares from Perth or Darwin cost around Aus$200 less. You’ll then need to add Aus$100–200 onto all these for the connecting flight to Prague. Return fares from Auckland to London range between NZ$2000 and NZ$3000 depending on the season, route and carrier.