Absolutely you can fly at night under the without an NVFR rating - it is one of the privileges of the CIR (CAO 40.2.1 Sect 14) providing you have the required currency for night landings.
So long as you meet all the requirements for the issue of a NVFR rating and your IR is current, you should be able to apply to CASA for the issue of a NVFR rating. Similarly, the holder of a CIR can apply for a PIFR rating which allows them to fly under the IFR without the currency requirements.
It costs you for each rating of course, but a couple of guys I know got their PIFR and additional Navaids and M/E added to their NVFR after CIR was completed.
When you say you have to remain "IF current", do you mean have a current rating, or you mean current to fly under the IFR? If you mean you are not remaining current to fly IFR (despite a current rating), then a NVFR rating won't do you any good because if it's IMC then you're stuffed anyway! You are still fine to fly NVFR as long as your rating is current, even if you haven't flown in IMC for months. Make sure you meet the requirements of Sect 14 for charter ops!!!
If you mean that you have to keep the rating - if it's charter work, won't your employer want you to have the rating anyway??