You don't need it to be in the by-laws … it's in strata law.
117 Owners, occupiers and other persons not to create nuisance
(1) An owner, mortgagee or covenant chargee in possession (whether in person or not), lessee or occupier of a lot must not:
(a) use or enjoy the lot, or permit the lot to be used or enjoyed, in such a manner or for such a purpose as to cause a nuisance or hazard to the occupier of any other lot (whether that person is an owner or not), or
(b) use or enjoy the common property in such a manner or for such a purpose as to interfere unreasonably with the use or enjoyment of the common property by the occupier of any other lot (whether that person is an owner or not) or by any other person entitled to the use and enjoyment of the common property, or
(c) use or enjoy the common property in such a manner or for such a purpose as to interfere unreasonably with the use or enjoyment of any other lot by the occupier of the lot (whether that person is an owner or not) or by any other person entitled to the use and enjoyment of the lot.
Call Fair Trading on 13 32 20 or go HERE to download the form you'll need to arrange a mediation – a compulsory first step before you can pursue a complaint at the CTTT. If mediation fails the next stage is to ask for an adjudication. You will need as much documented support as you can muster, including statutory declarations or notarised letters from neighbours and friends who have witnessed the smoking and its effects. A statement from your doctor that the neighbours smoking is affecting your health will also help immensely and, even if they don't want to take on the issue themselves, a letter of support from your Executive Committee will help too.
There has been a case in the past where the CTTT ordered strata residents to stop smoking. That is not a true legal precedent and the CTTT is notoriously fickle but it is an indication that you have a reasonable chance of success if you pursue it.
Smokers do have a right to do what they want in their own homes – but not if it adversely affects their neighbours.
In the long term, you should propose a by-law for your next AGM that prohibits smoking on common property or allowing smoke to pass over common property or into another owners lot. This is a standard by-law that most new buildings have as part of their basic health and safety measures.
Many smokers these days are aware of the offence and damage their addiction causes other people and take steps to avoid it. Anyone who doesn't is utterly selfish and inconsiderate and deserves no sympathy from anyone. This isn't about freedom or the “nanny state” – it's about basic human decency.