In 2017, he joined the Parramatta Eels, playing for their under-20 side.[12] In May 2017, he again represented the Junior Kiwis.[13] On 4 September 2017, he was named in the Holden Cup Team of the Year.[14] On 1 October 2017, he scored a try in the Eels' Holden Cup Grand Final loss to the Manly-Warringah Sea Eagles.[15]
In 2018 and 2019, Marzhew was a member of Parramatta's NRL squad but did not play a game for the club, instead playing for the Wentworthville Magpies, their NSW Cup feeder side.[16][17]
On 30 September 2019, he was named in the NSW Cup Team of the Year.[18]
In 2020, Marzhew returned to the Gold Coast, signing a two-year contract with the club.[19]
Marzhew played a total of 17 games for the Gold Coast in the 2022 NRL season and scored nine tries as the club finished 13th on the table.[23]
In November, Marzhew was released from his Gold Coast contract and signed a new three-year contact with the Newcastle Knights starting in 2023, in a player swap with Chris Randall.[24][25]
2023
In round 4 of the 2023 NRL season, Marzhew made his club debut and scored two tries for Newcastle as they defeated Canberra 24-14.[26]
In round 6, Marzhew scored two tries for Newcastle in their 34-24 victory over the New Zealand Warriors.[27]
In round 13, Marzhew scored a hat-trick in Newcastle's 28-18 victory over Manly.[28]
On 13 June, it was announced that Marzhew had been demoted from the first team squad after failing to meet club standards. Newcastle issued a statement which read “Greg missed the team bus early on Sunday morning, which is a clear breach of team standards and as a result he will not play this weekend".[29]
In round 25, Marzhew scored a hat-trick in the club's 29-10 victory over South Sydney.[30]
Marzhew made 22 appearances for Newcastle in the 2023 NRL season and scored 22 tries as the club finished 5th on the table. Marzhew played in both finals games as Newcastle were eliminated in the second week of the finals by the New Zealand Warriors.[31]
2024
Marzhew played 22 games for Newcastle in the 2024 NRL season as the club finished 8th and qualified for the finals. He played in their elimination finals loss against North Queensland.[32]