Howlett joins Rod Kafer and Brad Thorn in having won major rugby tournaments in both the southern and northern hemispheres, the Super 14 and Heineken Cup respectively. He enjoyed his success with the Auckland Blues in the 2003 Super 12 and Munster in 2008. With Munster winning the Magners League title for the 08–09 season, Howlett held both a Heineken Cup and Magners League medal in the space of just two seasons with Munster. On 26 February 2010 it was announced Howlett had signed a contract to stay on at Munster until the end of the 2010–11 season, with the option for a further year. That option was confirmed by Munster in February 2011, and he began the 2011/2012. But in December 2011, Howlett picked up an Achilles tendon injury in a Pro 12 game against Ospreys which ruled him out for the rest of the season.