A check of Google for Australian sites ending in .au, returns 1.76 million results for nighttime, 3.88 million results for night time (which includes night-time) and 101,000 for nightime.
Nightime should be considered a spelling error, but it is used for Codral Nightime Cold and Flu tablets, where misspelling a word is common in branding and marketing situations.
The compound word nighttime is a US spelling. In Australia the spelling is not listed in either the Macquarie dictionary or the Australian Oxford dictionary, so nighttime should be considered a spelling error in Australia.
The Macquarie dictionary lists night-time as a noun, with no reference to night time. The Australian Oxford dictionary also only lists night-time, again with no reference to night time.
I have to say I didn’t see that coming. The correct spelling for Australian usage is night-time with the hyphen.
A quick review of the first 100 sites for “night-time” shows only 30% of sites use the correct spelling, many using both night time and night-time in the one article which is considered an error. Given the number of people using the spelling nighttime, nightime and night time, it is conceivable that 80% or more of writers are using the incorrect spelling for Australia, according to the Macquarie and Australian Oxford dictionaries.
With Microsoft Word, if you enter nighttime Microsoft Word will mark nighttime as an error, but suggest both night time and night-time, which could easily lead writers to use the wrong spelling for Australia, depending on what they mean to write.
The preferred Australian English spelling.