Yes, you had a right to silence and were legally not required to answer the questions. A (capable) CYFS worker should have informed you of your rights, however as they don't really have the authority to be questioning you in the first place regarding a criminal offense, it may have been deemed unnecessary. However, I would recommend you pick your battles - if you believe you had done nothing wrong, answering their questions as opposed to the Police would probably be a lot less stressful in the long run, unless you felt as though they were attempting to coerce a "confession" out of you, in which case speaking up during their questioning may have become incriminating.

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I think if he got a fine he also got a conviction. Go into local police station and ask if he has any convictions, or just included it on the passport not all convictions are used to decline passports.

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While bicycles are included in the definition of 'vehicle' the drink driving section of the Land Transport Act refers to 'motor vehicle'. This is quite a significant difference. For a bicycle to be classed as a 'motor vehicle' it must be 'mechanically' powered.

While there may be an argument that turning pedals is a form of mechanical power it is not cut and dried. (However, cops can still confiscate your bike if you're operating it dangerously and good luck crying to a judge that your drunken cycling was perfectly safe :)

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You could go to jail for up to 3 months but generally it will depend on your previous driving record, if you have a pretty good record you will probably get a fine and your licence disqualified. Possibly worth talking to a lawyer represent you on this.
http://www.youthlaw.co.nz/page/Road+Rules this link outlines some penalties, scroll to the bottom of the page.

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