Is there a specific reason why Superman Returns was shot in Sydney, Australia? You can tell because some locations are obviously Sydney, Australia.
You could pose this question for many tentpole releases, and with regards to my own country (UK), the late 70's saw the beginning of a trend which saw many large studios invest in the UK due to the large stages, spaces, local production talent and financial incentives that the London studio lots offered over Hollywood and L.A., attracting many large scale fantasy projects from Star Wars and the Salkind Superman releases right through Alien, Aliens and the Indiana Jones saga to Batman in 1989.
According to Wikipedia, the films producers were already looking at Australia as Warner Bros. considered shooting Superman Returns at Warner Roadshow Studios in the Gold Coast, Queensland. After filming, this could have been used as an attraction for the adjoining Warner Bros. Movie World theme park, but the idea was scrapped for being too expensive.
When this initial plan went through, the Fox facility in Sydney (which was still virtually brand new at this stage and most likely was actively attempting to attract big projects) seems as good as anywhere else, especially if Warner Bros. had already started to invest in the country and its production talent.