There are also direct flights from New York to Sydney with an estimated duration of 21hrs 45 mins. However, you have to consider the fact that sometimes there are no seats left on direct flights between these two destinations or tickets are too expensive.
To fly from New York to Los Angeles would take approximately 4hrs 55 min and then the flight from Los Angeles to Sydney lasts around 16hrs 45 min. The connection time may probably be an hour and a half if it is one of those flights with a continuing service. The whole trip would last for around 23hrs 10 mins in total.
For people who would like to have a more exciting journey and a longer break from the flights might consider connecting flights with a stopover. Flights exiting New York (JFK) en route down under to Sydney have several exiting stopover options. Depending on your airline you can fly west across the Pacific for stopovers in Hawaii, Tahiti, Fiji, Hong Kong, Japan and New Zealand, or alternatively fly eastward and stopover in the Middle East at Abu Dhabi. This would be a real round-the-world trip with a flying time of around 27hrs.