Mr Hewitt's new job - overseeing the facility, its staff, and gearing up for the summer rush - is just the latest he has held there over the past few decades.
"Really I started at the very bottom as a volunteer and worked my way through," he said. "I've been an aqueous diver, feeding the fish and looking after the animals, right through to supervising, and now managing the whole building."
After getting hooked on the aquarium he spent the rest of his teenage years working casual and weekend jobs, even continuing while working in the commercial fishing industry.
Then nearly 30 years ago he was asked to apply for a job as an aqueous diver, and the rest is history.
Although his initial attraction to the aquarium – a love of animals – has not dulled, he said he stayed as there were so many new opportunities to take up within the one facility.
"There's always been interesting stuff happening, it's always had some ambition in it," he said.
"In any one day to be able to go diving, feed the piranha, to handle kiwi, or doing stuff with tuatara, it's just the dream for so many people.
"The initial major drive was the animals and the exciting things we do, just being able to go and dive and feed the fish and get paid for it, to looking after the animals and seeing them progress."
Over the years he has played a large role in ensuring the facility is a drawcard for tourists, to a place where children can learn more about the marine environment.
In more recent times he became the exhibits and animal health director, and had been able to get involved with the construction of new exhibits - including building the moray eel, rocky shore exhibition.
He recently took over the successful kiwi breeding programme, and was able to "give it a good shot in the arm".
Mr Hewitt said he and his fellow aquarium staff were "here to make the aquarium the best it can be and put our best into it".
"I couldn't have done it without the rest of the aquarium team. It's only because they're there to back me up that I'm able to do what I do. Without their support I'd really be struggling."
In 2002 he witnessed the expansion of the Marine Pde building into the facility it is today, meaning he knows its every inch: "I know where the pipes are and the power and all the interesting little bits and pieces that can be a struggle to find out if you don't know."
He knows what works best when setting up enclosures, or exhibits, to what design systems are the most efficient.
This experience could come in handy if the Napier City Council's proposed $45 million aquarium expansion with "high-profile" partners including Weta Workshop, Air New Zealand and the University of Waikato, goes ahead.
Napier mayor Bill Dalton said this project was still on track, with council preparing a business case while their indicative business case is before central government.
"This project can't proceed without government support. This is a monstrous project and it needs to be done absolutely correctly," he said.
"It's potentially a game changer for Napier. If we have got a facility in our city the size, and the importance we're talking about it will have a marked effect on the Napier economy and the Hawke's Bay economy, because we'll have people coming from all over New Zealand and the world."
The appointment of Mr Hewitt to acting manager meant the aquarium was "in good hands", he said.
Although he isn't sure what role he might play if the expansion happened, Mr Hewitt said it would turn the aquarium into an "amazing beast".
As well as being a "major environmental, conservational, educational hub for anything to do with the [marine] environment" it would place more emphasis on other animals – including the kiwi, and tuatara – and would offer research opportunities.
"So to bring all these students to Hawke's Bay as well as tourism, to have an expert area like this here is just great for Hawke's Bay, full stop."
As for his predecessor's "retirement", Mr Yarrall has taken a part-time role as a technical adviser to the aquarium, and as the skipper on the boat which collects specimens and food.
- The National Aquarium of New Zealand is open every day (except Christmas Day) from 9am to 5pm with last entry at 4.30pm. Daily feeding times are 10am for the reef fish and 2pm for the sharks. The little penguins are fed at 9.30am, 1.30pm and 3.30pm.