in and at are used for similar things in regards location. Sometimes they can be more or less freely interchanged, but other times one is used more typically than the other.
For example, you generally use in with cities, states/provinces, countries (regions)
I live in London
He used to live in California
She spent a summer living in France
None of these would typically have at in them.
at is used for general locations
He is at home
She is at work
It is also used for buildings and other smaller locations (points)
He is at the movie theatre
She is at the Empire State Building
It is more ambiguous or at least either could work in between these two extremes. Regions within a city or country could be in or at, partially dependent on whether they are thought of more like a point or a region.
He is staying in Sydney at Camden
in comparison to
He is staying in Sydney in Camden
The first sounds more natural to me, but it also treats Camden more as a single point. The second one implies Camden is a region; however, it also seems to imply Sydney is part of Camden (rather than the reverse). This nesting doesn't happen with at; It would be more natural to express the second sentence as
He is staying in Camden, Syndey
at doesn't have this nesting effect. That is,
He is staying in Sydney at Camden
does not sound like Syndey is a part of Camden. This would be because at is like a point and thus can't really contain another region.