His best known work is the Citizens' War Memorial in Cathedral Square, Christchurch, where the city's annual Anzac Day service is held.

He obtained knowledge of anatomy by observing the different muscles while he shaved, and read about the sculpting work of Gian Lorenzo Bernini, Michelangelo and Auguste Rodin.

At the time, the sculpture was judged to personify the Anzac spirit, and the mayor held an enthusiastic speech at the unveiling on 26 April 1922.

[2][5] His next commission was a memorial in Waimate for Margaret Cruickshank, the first registered woman doctor in New Zealand; she had died from the 1918 flu pandemic.

Trethewey worked from photos and sculpted a 3 metres (9.8 ft) tall statue from a five-ton piece of Carrara marble.

[2] The sculpture was unveiled on 10 August 1932 in Victoria Square by the Governor-General, Lord Bledisloe, and brought Trethewey much publicity, including coverage on Movietone News.

Two of those proposals, the other the Bridge of Remembrance, were eventually adopted, with Gould's idea receiving the support of the Canterbury Anglican elite.

While the Bridge of Remembrance was unveiled in 1924, the Christchurch City Council opposed the Cathedral Square proposal and stopped it from going ahead.

Gould seized the opportunity and proposed the vacated site for the memorial, and the Anglican Church as the owner of the land agreed under the condition that a cross be incorporated into the design.

[14] The design was accepted in June 1933, after which Trethewey refined it before carving the figures in clay, boxing them up, and forwarding them for casting to Arthur Bryan Burton's Thames Ditton Foundry in Surrey.

[15] According to MacLean and Phillips in The sorrow and the pride: New Zealand war memorials, it is possible to make 'a good case...for it being the finest public monument in the country'.

[16][17] The memorial is registered as a Category I heritage item with the New Zealand Historic Places Trust, and it is the site of the annual Anzac Day service in Christchurch.

[16] Trethewey was commissioned to produce a sculpture of Maui Pomare (1875 or 1876–1930), a prominent Māori political figure, which was erected in Waitara in 1936.

A 100 feet (30 m) frieze depicting the progress of New Zealand, groupings of pioneers, lions in Art Deco style, a large fountain and a figure of Kupe standing on the prow of his canoe were produced for the centennial exhibition.