The house flag of the Union Steam Ship Co. of New Zealand Ltd, Dunedin and London. A red, rectangular flag with a union flag in the centre, surrounded by the white letters 'USS Co'. The flag is made of a wool and synthetic fibre bunting. It has a cotton hoist and is machine sewn. A rope and toggle is attached.

James Miller, a manager of the Harbour Steam Consortium in Dunedin, New Zealand, took the opportunity to expand operations on the death of the owner. Following a lack of support in New Zealand his partner John Darling went to Scotland and persuaded Denny shipbuilders of Dumbarton to invest in the new business and build two new steamships for the company. Other Scottish investors came on-board, and together with additional New Zealand interest, the Union Steam Ship Company was born in 1875 with James Miller as managing director. For the next 25 years it expanded and absorbed any competition on the trade routes to Australia, Tasmania, the Pacific Islands and North America to create a near monopoly in both passenger and cargo transport.

In its drive to maintain its position, the company invested in coal mining to protect and ensure its coal supplies, and created a thorough system of documentation to ensure efficiency both on shore and on ship. This included the famous Red and Blue Books for masters and pursers which had instructions on what to do in any emergency. They also had a tradition of developing the latest technology for their ships, including the world's first ship to be built of mild steel, the 'Rotomahana' in 1879, and the first vessels to be powered by gas turbine in 1975, the 'Seaway Prince' and 'Princess'.

In 1917 the USC was sold to P&O, and the fleet was then controlled from London, though retaining its name and separate structure. In 1972, TNT, headed by Peter Abeles, an Australian transport tycoon, bought back the company from P&O with 50% of the shares guaranteed to be owned by the New Zealand government. The company modernized and moved into container and roll-on/roll-off shipping. In 1996 Brierley Investments Ltd took over and the Australian operation was sold off in 1997. In 2001 the shipping company closed in the face of increased international competition.