He's charged with keeping the All Blacks in optimum shape so they can stay on top of the rugby world.

And now the side's strength and conditioning coach Nic Gill has set his sights on helping everyday Kiwis get and stay fit.

His new book - Health Your Self - is promoted as a "one-stop handbook to a healthier, more energetic you".

Read the full book extract of Health Your Self.

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And it has a simple message that - just like All Blacks - ordinary Kiwis also need to learn to control their minds and choose good habits over bad if they are to succeed in their health goals.

This can even mean confronting those dearest to us, such as a loving mum or spouse who overfeeds you or a close buddy wanting you to drink one-too-many glass of wine.

"Each of us has different things in our life that make it difficult for us to stick to a healthy lifestyle," Gill writes in the book.

"It might be habits that we learnt from our parents; our financial situation; time pressures; the people we live or socialise with; or a voice inside our head that constantly tells us to take the easy option. "

Sonny Bill Williams with Nic Gill during an All Blacks gym session. Photo / Brett Phibbs
Sonny Bill Williams with Nic Gill during an All Blacks gym session. Photo / Brett Phibbs

"All of these are potential barriers to change – but as long as you're aware of them, you can overcome them."

And overcoming barriers is a challenge Gill knows plenty about.

Following another failed New Zealand Rugby World Cup campaign in 2007, Gill took over as All Blacks' head of strength and conditioning in 2008, helping to lead the team into a period of unprecedented success.

During his tenure, the All Blacks claimed Rugby World Cup titles in 2011 and 2015 and won more than 120 tests at a winning percentage hovering above 80 per cent.

And while ordinary Kiwis don't have to face up to hordes of Wallaby or Springbok tacklers, we do still have plenty of obstacles attempting to get in the way of staying healthy.

The first most "critical" step to overcoming these challenges is to find a strong inner motivation and desire to get and stay healthy, Gill said.

Next, the fitness guru identifies common challenges and solutions.

People who encourage your bad habits

Gill said these are often the friends you love spending time with who say, "Come on, let's get another bottle of wine", or "Right, we've walked for 20 minutes – we've earned an icecream".

It could also be your spouse who buys too many chocolates or Friday-night fish and chips because they want to please you.

While this is OK as a "sometimes" activity, it becomes a problem when its regular and interferes with your health efforts.

So what should you do? Maybe suggest a coffee instead of wine to your old friend.

After all, true friends aren't likely to be offended and you are still going to have a great time together, Gill said.

While possibly a littler trickier with your partner, this is the moment to use "good communication skills" to express gratitude for their kind thoughts but to try to also discourage them from doing it in the future.

People who show love by feeding you

Let's face it, this is most likely going to be your mum because providing food for family members is a fundamental human behaviour, Gill said.

One way to tackle this is to tell your mum you know she just trying to make you happy, but, at the same time, you don't want to eat too much because you want to be healthier and live longer so you are there for your parents when they are older.

Work issues

"I'm too busy with work', is probably the most widely used excuse when it comes to changing our health habits. And for most people, it really is just an excuse," Gill said.

The most simple way to find time to be healthy is to ensure you cook and prepare meals to take to work instead of watching TV when you get home and to take healthy snacks to morning teas and meetings rather than cakes and biscuits.

You can also try to exercise during your lunch break or to build exercise into your commute by getting off the bus a few stops earlier and walking the rest of the way.

Health Yourself, by Nic Gill, is published by Penguin NZ, is released on September 17 and has an RRP of $4