If you want to put a label on it, you can call it the Wellington, New Zealand, session. New Zealand is 2 hours ahead of Sydney, Australia, and (in northern hemisphere summer time) 16 hours ahead of New York. So, the normal business day has already begun in New Zealand, before the normal business day has ended in New York. That is, at 5 pm New York time, it's 9 am in New Zealand (during our summer time, when we are on daylight saving time, and New Zealand is on standard time).
But, I want to encourage you not to fixate on these so-called opening and closing times.
It can be argued that there are only 3 primary trading sessions: London, New York, and Tokyo. Consider this: the interbank network is open and busy 24 hours per day, and that's where the real action is in the foreign exchange market. When the business day is winding down in London, the big banks in that network transfer their open positions and open orders to their New York branches. And when the business day in New York winds down, those banks transfer their open positions and open orders to their Tokyo branches. And, finally, when the business day winds down in Tokyo, those banks transfer their open positions and orders back to London.
Lastly, I'm not sure that you're ready for this, but I'm going to take a chance with it.
Here's a different way of looking at trading sessions ---
If you find this just adds to your confusion, simply ignore it for now. It will still be here, when you're ready to have another look.