Hi, I will let a member of the committee answer the access question.
As an astro-landscape photographer myself I would like to advise you that there is a fair bit of light pollution from Christchurch at the observatory and recommend trying somewhere else for time lapse or milkyway photography. It may be fine if you are doing deep space objects, but if doing the whole milkyway I would recommend finding somewhere further afield or shielded from the Christchurch light pollution. The main place that others I know, who do landscape work, go to is Lake Ellesmere for the full effect and if you are lucky a spike in Aurora values which is visible from there. Godley heads is closer and gets a great view of the milky way rising from the sea. From the carpark there is a hill that shields the city lights. Though you can’t really shoot north from there. All the best for your shots. Trace