He's sold more than fifteen million albums and is worth more than $100 million. So what does Bieber have to say about fame, fortune and his grumpy side?

It's been fifteen months since we last saw Bieber, and it seems there have been plenty of changes in his life since then?

“The biggest change in my life is probably Believe. Releasing this album is probably the biggest moment in my life, and just the transition, and knowing that I'm maturing, and my fans are maturing,” Bieber said. For him it’s all about “finding the happy medium where my voice sits, which songs to release. It was tough trying to figure that out, but I think we really got it down perfectly.”

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Bieber is clearly maturing, both in his music and as a person.

“As a person I am definitely maturing - just getting older and having those responsibilities of being eighteen. And music, as far as what I am able to talk about, and what is appropriate for me to talk about now, being eighteen. I think it opens a lot of doors, and you know it's great because with this album I have heard a lot of people say ‘I never really listen to your music before and this album really caught my attention, and you know I am a fan.’ So to hear that, and see different guys that are coming to my concert, and older people, it’s like it’s definitely a well all rounded crowd, and I never want to lose my fans, but I just want to grow.”

When Bieber was here last April, Australia gave him an unforgettable welcome.

“I am never going to forget that,” he said. “The last welcome was incredible. I mean the fans, the hysteria. This time I think it’s different because in the airport I went out the back, and I have the car scoot me right when I get off the plane. So I go into the car and can drive right off the tarmac, so I don't have to go through the airport and get like attacked. Because last time I almost died and this time it was cool.”

But there’s no escaping the Beliebers. “They found out the hotel I'm at and they were screaming all night, it was crazy.”

Clearly used to plenty of craziness from fans around the world, Australia’s welcoming party is still up there in the high end of crazy.

“I think this time is even crazier, it’s just last time it was uncontrolled because we just walked through the airport,” Bieber said. “It was uncontrolled and that is why it was so much. This time it is more controlled and there are barricades, the security is well managed, but it is still just as crazy.”

So how does the teen heartthrob deal with that level of fame? He says it’s by following some simple advice from those who’ve been there before him. “Just to stay humble and keep your family close. Keep the people that were there at the beginning there, because those are the people who cared and care enough to stay with you.”

Bieber has a close relationship with his mum, which he says he maintains even while jetting around the world.

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“The relationship with my family … you know I keep that first. Whether it's flying my grandparents out like last time I was here my grandparents were here with me, or my dad and little brother and sister and my mum. We all are so close, and that's what keeps me humble, and I do this for them - it's awesome.”

Keeping Bieber grounded is all about not letting the fame get to his head.

“That's what family members are for. You know whether it's like ‘Justin, you need to pull up your pants’. That's what my grandparents say all the time. ‘Pull up your pants Justin’. My mum is always saying something, but they are always super proud of me, no matter what,” he said.

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Image is a big thing for Bieber and his brand. But is it something that’s hard work for him, or does it just come naturally?

“I just express myself and I am not afraid to do that, whether it’s you know whatever I wear, I always make sure it’s me,” Bieber said. “If I wear a certain thing, like I wore a headband on my head frontwards, and people were like ‘a stylist is playing a trick on him’. Like I don't care if this person is writing something about what I am wearing, that's how I want to express myself no matter what.”

So how does he cope with it all - the endless lights, camera, action and paparazzi?

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“It’s part of it, not everything is going to be perfect. I don't love interviews, I can't tell you that. But it is a part of what I do, and you know I just accept it I guess. You have got to do them.”

There is plenty of talk about how much money the Bieber brand is generating through album sales and other projects. But is interested in becoming more of an investor and a business person?

“Well I think it’s just about making the right business decisions and having those people in place you can trust to, you know, to help with your finances. I have a great business manager and definitely people to help me make the right investments and stuff, and I am always super careful of who knows - even my team, nobody knows about my money and I keep it really secretive.

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Bieber’s documentary film - Never Say Never - generated about $30 million, and was the highest grossing documentary ever in the US.

But a little known fact about the film is that Matt White and Today Tonight make a short cameo. Unfortunately though, Bieber won’t discuss any royalties.

Getting ready for another performance with Sunrise, Bieber is excited.

“I am looking forward to it 100 per cent. I always love coming here and Sunrise have always been nice to me, so it’s going to be awesome.”

So does he ever get tired and grumpy of the performances?

“Of course, of course, I mean I am a human being. People think I'm a battery you have to turn on, but sometimes that battery gets empty and I need rest. My team knows when I am tired and how much I can handle by now.

But what exactly does the grumpy Justin Bieber do? Does he shut down? The grumpy Bieber is easy to see - all it takes is a few pushy questions about his personal life, and his girlfriend.

Grumpiness and jokes aside though, being Bieber is pretty fun.

“I love travelling, I don't like the actual travelling part and being in the planes and stuff, but like seeing different places and visiting - I love that. I love twitter and I love that fact I can tweet 25 million people whatever and they'll listen, that's a really cool thing as well.”

How important is social media for him - not just personally but also professionally?

“Definitely it’s a big deal. I was able to have that social media connection with my fans and have them reach out to me through Facebook and Twitter, and YouTube really launched my career,” he said. “For fans to really feel involved with me, and to comment - it was something different and you could not do it ten or twenty years ago.”

With 25 million followers now on Twitter, Bieber is hot on the heels of the reigning Twitter queen Lady Gaga.

“I'm trying, I'm trying to get number one, but Lady Gaga is still there,” he said, also promising to follow Matt White on Twitter.

With a full schedule during his visit to Australia, there is one thing that Bieber would love to do if he gets some spare time

“I want to see if they have those shark cages where I can go in and have like sharks swimming around me while I'm in the shark cage - that would be cool.”

Instead of swimming with sharks, Bieber is appearing on Australia’s Got Talent. “It’ll be a honour for me. I mean I saw some of the contestants and I think they are really talented, it’s crazy,” he concluded.

Bieber fever continues tomorrow with his huge live performance at Sydney's Circular Quay for Sunrise, which starts with Mel and Kochie at 6.

This reporter is on Twitter at @MattWhite_7