Recruited in the 2000 AFL Draft at pick number two, there were high expectations on Koschitzke from a young age. His athletic 197 cm build and strong marking were a great asset to the St Kilda Football Club, which drafted him and gave him his AFL debut early in 2001. He made an immediate impact and, in a debut year where he played 20 games, he won the AFL Rising Star award.

In 2002 Koschitzke played just four games in an injury-riddled season. His quad and hamstring injuries continued to cause him trouble into 2003 and 2004 when he missed five and eight games respectively.

In 2005, another injury-affected season, Koschitzke played sporadically until he made a return in Round 14 against the Western Bulldogs. In a best-on-ground performance he picked up 19 disposals and kicked three goals. His form continued when he kicked 16 goals over the next four games, but injury struck again after Round 21, causing him to miss the finals series.

In 2006 Koschitzke suffered further injury problems, first missing Rounds 1–3 with a minor knee injury and then missing Round 5 with a quadriceps injury. After being a surprise inclusion for Round 6 against the Bulldogs, he was knocked out by Daniel Giansiracusa with a legal hip and shoulder. He was diagnosed with a fractured skull and was expected to miss four to six weeks. But four weeks later he had not regained hearing in one ear and, for future health concerns, was ruled out for the remainder of the season. Coach Grant Thomas later said that if he did return to play in 2007, it would be an unexpected bonus.

On 18 June 2006, Koschitzke collapsed while appearing on a live television spot for Channel Seven’s program Sportsworld while being interviewed by David Schwarz. He was released from hospital later in the day after scans found nothing abnormal. St Kilda staff put the faint down to a number of factors including, fatigue, dehydration and a minor virus. There was no direct connection with his original injury and the collapse.

Koschitzke played 19 games in 2007, both in the forward line and the ruck, kicking 26 goals (season high four goals twice) and getting 155 hit outs (season high 21) and 116 marks (season high 10 twice).

Koschitske played in 20 of 22 matches in the 2009 AFL premiership season in which St Kilda qualified in first position for the 2009 finals series, winning the club’s third minor premiership.

Koschitzke had one of his most consistent seasons in 2009 playing mainly at full-forward. He contributed to the Saints making the 2009 AFL grand final and polled well in the Brownlow Medal, finishing equal 11th.

Koschitzke played 21 games in 2010, including four final matches, and kicked 30 goals.

He married his long-time partner, Alicia Hampton, in January 2012 and together they have a son.