Hi Rod
Some gasfitters leave them with one bottle turned off so that  you have to manually switch  over to the other bottle and therefore know when to order another bottle rather than run the risk of suddenly having no gas.  However that said, the regulators that are fitted are auto change over ones.  You can visit www.ongas.co.nz and have a look around their site.  There is a good explanation of regulators and how they should change over in nice layman's language, and what to look for when the bottle is empty.  My advice is to talk to the builder and get him to talk to the gasfitter - it should be under warranty - if it is the reg at fault, otherwise the gas supplier's truck driver may be leaving one bottle turned off when he delivers and connects. 


The other possibility it that you have a faulty reg (not very common in such a new install) and it will be under warranty.  You should have a copy of a gas cert for the job which will have the gasfitters name - depending on the sort of contract you have it might just be a call to him.