Krejza is an all-rounder, contributing to the team mainly as a right arm off-break bowler, but also as a right-handed lower-middle order batsman. He became a regular in the NSW team in 2004/05 but lost his place in 2006/07. However solid form for Sydney Grade team UTS-Balmain led to his inclusion in the NSW limited overs team towards the end of 2006. On 21 December 2006 Krejza announced that he would leave the New South Wales Blues to join the Tasmanian Tigers due to being considered 4th choice behind spinners Stuart MacGill, Nathan Hauritz and Beau Casson in the selection process.

After crossing over mid-season to Tasmania , Krejza played in 2 Pura Cup games, claiming 5 wickets and scoring a disappointing 17 runs in two innings. In the Ford Ranger Cup competition, however, he made his mark, taking 8 wickets in 4 games whilst also scoring 83 runs, with a top score of 53 not out.

During the off-season, his cricketing career was placed at risk after being caught speeding and drink-driving by Hobart police. He was subsequently suspended from Tasmania's pre-season training and warned to stay away from alcohol by his team's leadership group.

His renewed dedication to cricket saw him excel in the 2007-8 season. In the Pura Cup, he played 7 games and scored 289 runs at an average of 36.12, with a high score of 63. He also took 18 wickets at 47.11. The only downside to his season was that he did not break into the one-day side, with incumbent Xavier Doherty being Tasmania's sole spin bowler in the one-day arena. He played only one game, and was named 12th man in Tasmania's competition victory against Victoria.

2008–09: Test berth in India

After being named to Australia's A Team tour to India in August, he was selected for Australia's Test tour of India. With his selection, Australian chairman of selectors Andrew Hilditch said, "Jason Krejza had a good season for Tasmania last year but is a selection very much for Indian conditions. The selectors felt right-arm finger spinners would perform well in India and Jason now has a chance to prove himself at the international level."

Krejza was seen as the second spinner in the squad behind Bryce McGain. However, McGain was sent home injured. Krejza was attacked by the Indian Board President's XI—a team consisting mainly of young emerging players or fringe Indian representatives—in a tour match, conceding 0/199 in 31 overs, and he was then excluded from the first three Tests at the expense of McGain's replacement, Cameron White. This was despite captain Ricky Ponting frequently hinting that Krejza would play and predicting that India's senior batsmen would not have the gumption to attack him.

Krejza eventually made his debut in the fourth test in Nagpur, which Australia had to win in order to draw the series and retain the Border-Gavaskar Trophy. He came in at the expense of fast bowler Stuart Clark and played alongside White. Rahul Dravid became his first wicket. He went on to claim 8 wickets for 215 runs in the first innings.^[4] In the process, he became just the fourteenth Australian to take five or more wickets in an innings on debut, and also created the unenviable world-record of conceding the most runs for any bowler on debut, while also taking most wickets in his first Test match innings. It also beat his best first-class bowling of 4 for 91.^[5] In the second innings he took a further 4 wickets for 143 runs, leaving him with figures of 12 wickets for 358 runs in his first test. He is the second on the list of Test bowlers who have conceded most runs in a single Test match, beaten only by Tommy Scott's match return of 9 for 374, for the West Indies against England in Kingston, Jamaica, in