The 23-year-old Mariu's remarkable consistency earned him a New Zealand ODI debut against Pakistan towards the end of the previous home summer. In his second match, he made a fairly smooth transition into top-flight cricket with 58 off 61 balls in Mount Maunganui.
Mariu has always had the potential - he was New Zealand's highest run-getter in the 2020 Under-19 World Cup. More mature now, he has found a method to pile on the runs in red-ball cricket too.
"I think I found a good formula for batting last season," Mariu told ESPNcricinfo on the sidelines of a camp at the Chennai Super Kings Academy in Chennai in June. "Obviously, with cricket you go through patches of scoring runs and not scoring runs. But I think I just found a good base and sort of worked through that and managed to find some success through that.
"Then it was just about staying level, I guess. I was understanding that I'm not always going to be scoring lots of runs, so just trying to stay consistent with it even if it's not successful. But at that time, I was lucky that it went well."
Mariu credits mental-skills coach John Quinn, who has also worked with Rugby NZ and Black Sticks Men (hockey team), for his success in the previous season.
"Yeah, I think it's mainly down to my mental routines and all that I've been working on," Mariu said. "I work closely with John Quinn back home - a mental skills coach - and he's really helped me sort of find a routine that I can repeat, and it just keeps me consistent. And when I am tired or things are changing, then I can just go back to that. It's helped me play long innings as well."
Mariu hails from a cricketing family. His father Marcus and older brother Josh represented Canterbury at the Hawke Cup level. When he was younger, Mariu had even operated the manual scoreboard for Canterbury matches at Rangiora's Mainpower Oval. Years later, he's troubling the scorers with his prolific batting for Canterbury.
"Dad played a little bit and it's good playing with my brother," Mariu said. "Obviously, I sort of just followed him [his father] in terms of choices to play cricket, but I was always [watching cricket] on TV and playing in the backyard, so it was good fun.
"We would just sit up on the balcony and spend most of the day, especially four-dayers, just watching games and putting the numbers and names up, which was cool at the time. We just got paid 50 bucks a day [for operating the scoreboard], but it was just really good fun spending days with him [my brother] and some of our mates would also come down."
Like most New Zealanders, Mariu played a lot of sports while growing up, cycling through cricket, rugby, football and hockey. He believes that his hockey background has had a positive effect on his cricket.
"At this camp, I've worked on hitting straight, which is what I wanted to do coming over here [...] Just keeping everything a bit more square-on and finding ways of hitting down the ground. So, that's been the main work here"
Rhys Mariu on the experience in Chennai
"Yeah, I think I've always been decent at sweeping just because I think that comes from playing hockey at school. That [sweep] has always come naturally to me."
Given his long reach and strong base, there are shades of Daryl Mitchell in Mariu, especially when he sweeps and reverse-sweeps. It was only fitting that Mitchell had handed Mariu his maiden international cap.
"Yeah, it was pretty special with Daz presenting me that," Mariu said. "He's been really, really helpful with Canterbury and then making the jump up. He's always let me know that if I have any questions or need anything, I can go to him. So, it's nice having someone like that in your corner."
Mariu's golden run last season included a career-best 240 in just his ninth game for Canterbury and first as captain, against Central Districts at Saxton Oval. His mammoth score led Canterbury to a ten-wicket win.
"Yeah, it was a decent CD attack," Mariu recalled. "I think the conditions were sort of in the favour of the batting team on those couple of days. And I think we just found good partners through that. Like, [Matt] Boyle was really helpful. He obviously had a hell of a knock there as well [116 from No. 4]. It was a young team and there was a really good vibe around for the whole week. Things just sort of fell into place in that game."
Mariu, however, wasn't satisfied. He's always hungry for runs and improvement. During the New Zealand winter, he travelled to Chennai and focussed on holding his shape for long enough and hitting the ball down the ground.
"At this camp, I've worked on hitting straight, which is what I wanted to do coming over here," Mariu said. "Sri [Sriram Krishnamurthy, former NZ pathway coach and current CSK Academy head coach] has been really good. Just keeping everything a bit more square-on and finding ways of hitting down the ground. So, that's been the main work here."
In the recent past, Canterbury have supplied a number of players to the Black Caps, including Will O'Rourke, Zak Foulkes, Mitch Hay and Chad Bowes. Mariu draws inspiration from his domestic team-mates and hopes to emulate them.
"Fults [Peter Fulton] and Brendon Donkers [the Canterbury coaches] have created a good environment," Mariu said. "With a lot of Black Caps being churned out, success breeds success. It's cool seeing those guys go up to the next level. It makes it feel like it's less of a jump because you spend a lot of time with those guys and then you go see them play up high. It's cool and it doesn't make it seem too far away."
Mariu's next assignment is an A team tour of South Africa, which comprises three one-dayers and two four-dayers. If Mariu can maintain his consistency, he might not be too far away from breaking into the Test side either.