Goldfish are actually a bad choice for mosquito control. Even though the health board recommends them, DOC recommends native fish as their studies show that goldfish are ineffective especially as the goldfish get older.
I'm thinking that danios or mountain minows would be a better choice compared to goldfish or trout, as they are a smaller fish which will prefer smaller meals (ie bugs). And they are also more likely to breed and create a self sustaining population, the hundreds of goldfish that were released a few years ago have died out even though there's plenty of bug life. I'd love to use natives but I'm not sure which ones would be best.
The pond is actually an old building that is set below ground level in a concrete pool type hole. Rainwater used to be pumped out but now it is disused. There is no current or circulation and it is only filled by rain water. During summer it drops to about 6 ft deep (you can just see the top of the door). There is a 2 metre gap between the building and the sides of the pond. And the building is permanently flooded, which creates shade for a fair portion of the surface.