Nik Weyel's day was made when a rare 10-metre long whale shark swam past his boat.

The skipper of the Bay Explorer boat was taking 19 passengers out to spot dolphins in the ocean about 11km off Mauao when they spotted the whale shark about lunchtime yesterday.

"We were out looking for dolphins. We had a pretty bad run today. So there weren't many dolphins out there," he said.

"But then we came across this fin. I instantly thought it was a hammerhead [shark] because we see lots of hammerheads."

But Weyel, 24, wasn't too sure.

"I told the passengers over the PA system it was a hammerhead, maybe two. It is a massive shark whatever it is. I had never seen anything like it."

Weyel said the whale shark swam right up to the boat before going under it - that was when he got a closer look.

"I looked over the side and I just saw something big with spots on it."

Weyel asked his crew Heidi Glimaker what it was and she stuttered the words: "It's a whale shark" in surprise.

The whale shark was about 10 metres long. Photo / Supplied

"She couldn't even get her words out. We were so excited.

Weyel said he had only heard of two other whale sharks in Tauranga waters.

"It is unbelievably rare. They are the largest fish on the planet and they are super rare.

"It was quite an experience to come across that in the water and it was right underneath me."

Weyel believed the whale shark was about 10 metres long. "It is easily half the length of Bay Explorer which is 18m."

He believed whale sharks always arrived at this time of year when the water was warm enough.

The skipper had been working for the company for about two years and said seeing the whale shark yesterday was the "most awesome thing I have ever seen".

He had seen a rare leatherback turtle in Tauranga in a similar area to where the whale shark was swimming and noted the great white shark spotted in the water the other day.

"It really goes to show what a unique place it is."

Weyel said he and his crew were lucky to see so many different marine life day-to-day.

"But this absolutely took the cake for the coolest thing ever. It is just so rare."

Tauranga kayaker and marine conservationist Nathan Pettigrew was on the Dolphin Seafaris boat when Weyel called him over to see what was swimming beneath them.

"The passengers on board were over the moon," he said. "It was easily 10m long. It was incredible."

Pettigrew said whale sharks were uncommon in New Zealand.

"But we do get them here. We have seen more of them because there are more people with phones and cameras to report them.

"But to put it into perspective, I have been doing this for years and it is the first I have seen a whale shark in New Zealand."

Pettigrew said manta ray and whale sharks were often swimming in the warmer waters at this time of year.

"We hadn't seen dolphins at all that morning so to come across this whale shark was just incredible.

"It is the largest fish in the ocean. To see a manta ray and a whale shark in the same day is bucket list stuff."

His advice for people out on the water was to respect the marine life.

"Respect sharks, get to know them, learn about them. They are incredible."