In the absence of Steve Smith, David Warner and Cameron Bancroft from the Australian Test team following the 2018 ball-tampering scandal, Harris gained his first opportunity to play Test cricket for Australia. In a Sheffield Shield match against New South Wales in October 2018, Harris helped Victoria to recover from a poor start of 2/6 with an unbeaten 250, the best score of his first-class career. This innings put him into contention for one of the vacant spots at the top of Australia's batting order. In the first four matches of the Sheffield Shield season, he scored a further two half-centuries and averaged 87.40. After 25 first-class matches for Victoria, Harris had scored five centuries and averaged 47.58, and as a result of his improved consistency he was selected in Australia's Test squad for the first time.