If you are not a licensed plumber then it is illegal for you to touch it. Get a plumber. 

hi there, best to get a plumber in as you have a mains pressure cylinder that i bet has not been serviced since it was fitted.
while a joint may be leaking, it could also mean that the cylinder is unrestrained and has been moving about, or that the thermostat is faulty and the water is over heating the cylinder,
has the relief valve been serviced? it has to be flushed every 6 months?
regards

Thanks yeah house is about 10 years old we been here 7 yrs and hasn't had the 6month relief valve flushed till the weekend when decided to read those stickers on the cylinder  when I did there was some leaks coming out also but nothing when shut.

Hi Guys,

A related question - my mate's Mains Pressure setup - a DIY install, bought second hand off TradeMe.

What are the penalties for installing your own Mains Pressure HWC, and where can I find these to show him?  It's always bothered me, so it might be good to have some proper ammo next time I go round there. (I know Valve Regulated Hot Water Cylinders can explode with enough force to demolish your house, so I'm pretty wary about them. If it was up to me, I'd have even more compulsory safety features, but I guess they don't blow up that often. Any NZ cases, anyone know?)

(And this is not me asking for myself, trust me!)

hi guys, this from in-brief, cheers
...
In a recent prosecution, an Auckland man was convicted and fined $1000 plus court costs and solicitor’s fees, for illegally carrying out sanitary plumbing.
xxxxxxxxxxx installed a domestic hot water cylinder at a home in Auckland without an authorisation, and failed to set it to an adequately heated temperature. This created the potential risk of Legionella or other bacteria affecting the domestic hot water supply.
 

xxxxxxxxxxx installed a domestic hot water cylinder at a home in Auckland

Bummer of a name this guy has...!!!