William was an Ayrshire Boy, boy immigrants who were brought to New Zealand before the outbreak of WWI. They were brought out to alleviate a shortage of farming boys and were to be trained in all aspects of farming life as well as clothed and fed. Their wages were to be paid to the Labour Department while they were given weekly pocket money (aka 'slave labour' for the government)

William John Frederick Marshall was 19 when he arrived Jan 1914, a birth year of June 1894, but, as you say, he died 26 Aug 1937 and listed as 40 which would make a birth year of 1897 and that would mean he was only 16 when he arrived.
The Ayrshire and Suffolk boys were from all over the United Kingdom, rather than just East London and Liverpool. Do you know where he was born?

"World War One Reserve"
Surname: MARSHALL. Given Name: William John Frederick
Category: First Reserves
Last NZ Address: Wairau Valley Marlborough
Occupation: Station Hand
Recruiting District: Nelson.
1938 NZ Enroll: Wairarapa, William John Frederick & Selina MARSHALL c/o Frank Tutty, Gladstone, labourer and married.

He was a member of the Loyal Masterton Lodge, M.U.,

Wairarapa Daily Times, 27 Aug 1937
The death occurred at the Masterton Hospital yesterday of Mr W. J. F. Marshall, a former well-known Gladstone resident. The late Mr Marshall, who was about 40 years of age had been employed for the past five years by Mr F. Tutty, "Kingsland," Gladstone. Last January he met with an accident and was a patient in Masterton Hospital for some time. About a fortnight ago he became seriously ill and was again admitted to hospital. He is survived by a widow and three young children, to whom deep sympathy will be extended in their sad bereavement. The funeral is to take place to-morrow, leaving Mr G. Hyde's Chapel, Church Street, at 2.30 p.m. for the Masterton Cemetery

William & Selina married in the Wairarapa on 2 July 1932. Their first of 5 children was Richard John Marshall (1933-2014). Selina was born 12 July 1908 in Miki Miki, Wairarapa. She died 11 Feb 1973 and is buried at Clareville, Carterton