He was at Otematatta Station at end of 1874 where he spent some time as a cadet as a protégé of James Ritche of Dunedin, a friend of his Uncle John Leslie. In August 1877, on the advice of James Ritche, he visited Australia to look at Stations over there but did not very much care for Australia and returned to New Zealand in November 1877.

In April 1878 he purchased Grays Hills Station in Mackenzie Country in the centre of New Zealand's South Island (Lat 44 ½ South and Long 170 ¼ East) in partnership with a man called Fletcher. The run commenced where Tekapo river joins the Pukaki river on the east side of the Tekapo and runs north to within about 8 miles of the Tekapo Lake. It was about 24 miles long with greatest breadth of 7/8 miles with no neighbours within a day's journey. At one stage it had 19,500 sheep. Sold out in 1881 when wool prices were good, also possibly due to a dispute with his partner Fletcher, and went home to Scotland. This was considered to be a bad move, as this was prime sheep country. However the winters had been appalling.

He returned to New Zealand early in 1882 and, early in 1883 bought Aviemore Station in partnership with his friend, William McIntosh Murray of Adelaide, Australia. Before settling down at Aviemore, and since it was winter time in New Zealand, he again went home to Scotland on a visit lasting some months. He returned to New Zealand at the end of 1883 and established Aviemore Homestead near Omarama. In 1886 he arranged for house to be built and the station had 18,770 sheep and William Shirres had to ride 100 miles to take delivery of further 2,000 sheep. In June 1888 his sister Christian from Scotland visited him at Aviemore. Christain Selina Shirres's book gives an indication of his lifestyle whilst at Aviemore Station, her book also includes many letters from William Shirres whilst in New Zealand:

Aviemore station still has many stone structures that Les's father had built. Probably the largest set of stone sheepyards ever built in New Zealand still exist and are still used today. Some other walls still exist including dipping yards and race, woolshed and homestead. Aviemore is owned by Munro's who are also distant relatives. At Aviemore the stone yards are high enough and are used for cattle. In 1995 one of Ray Menzies (Ahuriri Downs) 3 wild steers jumped out of the yards and swam across Lake Aviemore. Stone buildings still exist at Georgetown in a very reasonable state. Stonework and construction work are of obvious Scottish origin but who was contracted for the work and when it was completed is unknown.

At the end of her visit he surrendered the lease (7.9.1888) due to the devastating snows, collapse of wool prices and rabbits. He then returned to Scotland with her on S.S. Tainui leaving Lyttelton, New Zealand on 20.9.1888. In his book (p 109-112) "Early South Canterbury Runs" (ISBN 0 589 006169), Robert Pinney suggests that a further factor was that William was an educated, rather than a practical man.

On his return to New Zealand in 1889 he married Selina Ann Munro, a daughter of William Grant Munro, a Highland who had migrated to New Zealand and bought a farm in Georgetown, in the plains which he farmed, until eventually after a long illness, of heart trouble caused by his asthma, he died in 1898.