Hi,
When we applied online for our children's citizenship last year- we got their certificates really quickly , think it was a couple of weeks from applying.
So you could upload those documents once you receive them as I guess it won't take long if you did it online.
However, if you lodge the application and they request the citizenship forms then I think they only give you 28 days to provide the documentation - so if you have any doubt as to whether the citizenship certificates will arrive in that time then you'd have to factor that in.