I want to take a photo of an object, in front of the Harbour Bridge at night.

I've taken a few night shots of the bridge and the opera house before (so I am aware I'll need a tripod, slow shutter speeds and a large depth of field). But I've never taken night pics of an object with big things in the background and googling hasn't helped so far.

Is anything different when there's a foreground subject added to the mix of a traditional night bridge shot?
Will the available light from the long exposure be enough to illuminate the object, or do I need to use a flash so the object in the foreground is illuminated?

I'm asking because I don't know if I'll have time to do test shots, before I need to do this for a group and I'd hate for none of the pics to work.

I assume if I do need a flash, I'll have to buy one, as the camera flash probably won't be effective enough - is that right?
(I have a tripod, a Nikon D70s and a Nikkor 18-70mm lens)