There are 3 huge, multinational trading sessions – the Asian Session, the European Session, and the North American Session. These are commonly referred to as the principal trading sessions. In my opinion, it makes sense to view the overall market in terms of those 3 sessions, rather than sub-dividing it into smaller, regional, or national “sessions” (for example, Wellington or Singapore, which you mentioned).
Here are the elements that make up each of those three principal trading sessions:
(1) The Asian Session (traditionally referred to as the Tokyo Session) includes the most active portion of the Wellington and Sydney markets, plus all of the Mainland Asia markets (Singapore, Hong Kong, Shenzhen, Shanghai, and Taipei), the Seoul market, and the Tokyo market.
Years ago, Tokyo was the largest of these markets. But now, Singapore and Hong Kong are both larger (in terms of trading volume) than Tokyo. Nevertheless, it’s customary to refer to the entire APAC (Asia/Pacific) market as the Tokyo Session.
The smallest market in the list above is Wellington. And for that reason, I don’t think that Wellington and Frankfurt should appear in the same list of sessions, as if they were somehow comparable.
(2) The European Session (traditionally referred to as the London Session) includes 40 countries and principalities in the central European time zone (Germany, France, etc.), 10 countries in the eastern European time zone (Finland, Ukraine, Greece, etc.), and 3 countries in the GMT time zone (the U.K., Ireland, and Portugal). This Session is the 600-pound gorilla of trading sessions, being much larger than either the Asian Session or the North American Session. And within this European Session, London is, by far, the biggest player, in terms of trading volume.
Although the 43 countries in the European Session extend over a vast geographical area, only 2 hours separates the far-eastern region (Greece, etc.) from the far-western region (the U.K., etc.), which is why all of these countries taken together are considered to comprise one trading session.
(3) The North American Session (traditionally referred to as the New York Session) includes the U.S., Canada, Mexico, Brazil, and several smaller South American markets.
The countries of North and South America are spread across 7 time zones (from the eastern tip of Brazil, to Alaska in the U.S. northwest), but financial activity across this vast area is heavily keyed to the normal business hours in New York (8 am to 5 pm ET). Thus, when the business day winds down at 5 pm in New York, forex trading across the entire continent (and, indeed, around the world) slows to a crawl.
John Kicklighter has written a good ARTICLE for Investopedia on the logic behind a system of 3 trading sessions: Tokyo, London, and New York. And he also addresses the question (which you asked) of which currency pairs are most actively traded during each session. I suggest you study his article.
I’ve been trading forex for a while now, and I’ve experimented with various systems of trading sessions:
7 sessions, 5 sessions, 4 sessions, and 3 sessions. In my experience, the 3-session system is simplest and works best.