The 23-year-old enjoyed a strong debut campaign with Hull in 2016 and as a result earned a place in the Super League Dream Team.
The Tonga international scored 14 tries in Super League and helped the club to a first-ever win at Wembley in the Challenge Cup final.
Fonua admits that this term will provide further challenges and he has been impressed with the shape of the squad after linking up for the start of pre-season training last week.
“It will be a challenge for us, so we’ll have to prepare properly in pre-season,” Fonua said.
“The boys have come back in really good shape.
“I think that because of how well we went last season - nobody wanted to come back too out of shape, so most of the boys will be able to hit the ground running.
“It’s so good to see that the boys are certainly ahead of where we should be in terms of our fitness at this time in the year.”