Southbound turnover through Stock Connect is running at about HK$86 billion a session this month, and the exchange publishes the arithmetic behind it. HKEX posts the buy leg, the sell leg and the trade count for every session, separately for the Shanghai and Shenzhen boards; the twelve trading days from 1 to 16 September add up to HK$1.03 trillion of gross turnover. The net mainland investors finished that stretch with — buys minus sells, the figure the daily quota is measured against — was HK$32.8 billion.
The ledger HKEX actually publishes
Two files matter here and both are free. The historical daily page carries one file per session, per board: gross turnover, buy turnover, sell turnover and the trade counts that go with them. The historical monthly file does the same for the month, and the two reconcile. For Wednesday 16 September, the most recent session on this desk's screen, the Shanghai board recorded HK$49.5 billion of southbound turnover — HK$26.1 billion bought, HK$23.4 billion sold — and the Shenzhen board HK$29.3 billion, with HK$14.3 billion bought and HK$14.9 billion sold. That is HK$78.8 billion across and HK$2.1 billion left net.
The boards are not interchangeable and the files do not merge them. On that Wednesday the Shenzhen side finished with sellers slightly ahead while the Shanghai side finished with buyers ahead, and a reader who takes only the combined figure sees neither.
Gross, and what is left net
Because the numbers arrive in two legs, gross and net can be made to tell opposite stories, and both would be honest. For every HK$100 of stock that crossed the border in those twelve sessions, roughly HK$3 was left bought. The rest was the other leg, mainland accounts selling Hong Kong stock — the buyers of it may sit on either side of the border, and HKEX does not say which. Southbound is a two-way market with a mainland address, not a one-way pipeline, and the enormous gross figure says more about turnover than about conviction.
Eleven of the twelve sessions finished net buy. The exception was Friday 4 September, which was also the month's busiest: HK$106.7 billion across, and HK$10.0 billion more sold than bought. Why that particular Friday flipped is not reported here. HKEX publishes flows, not reasons, and a desk that invents a reason for one session's turn is writing fiction with a number attached. The same discipline applies to the three days since Monday: HK$72.5 billion, HK$74.6 billion and HK$78.8 billion of turnover, each net buy, each small.
The month the pace came off
August, the last complete month HKEX has published, was slower than July. Southbound average daily turnover came in at HK$107.3 billion across 21 sessions, 17.0 per cent below July's HK$129.2 billion, according to the exchange's Monthly Market Highlights. The northbound side of the same table fell 18.9 per cent, to RMB 300.0 billion a day. September has not recovered the pace: at HK$86.2 billion a day it sits 19.6 per cent below August's.
Set a year of monthly files side by side and the shape is clearer than any single month. Net buying is the norm — eleven of the past twelve months ended with buys exceeding sells, the one exception being May, when southbound finished HK$3.6 billion net short. What is thinning is the residue rather than the flow. August's HK$10.4 billion net was the smallest net-buy month in that window, and this month's HK$86.2 billion daily pace is below every month of the past year except last December's HK$83.6 billion.
What mainland money was actually trading
The names are published as well — the ten most-traded southbound lines on each board, every session. On 16 September a single stock topped both lists: Z.AI (02513), at HK$4.84 billion of turnover on the Shanghai board and HK$3.06 billion on the Shenzhen board. That is HK$7.9 billion, or about a tenth of the day's entire HK$78.8 billion southbound gross, in one line.
The rest of the Shanghai top ten read YOFC, MINIMAX-W, SMIC, KB Laminates, CIG, CNOOC, ZJ Innolight, Tencent and Zhida Tech. Shenzhen added Hua Hong Grace and Genscript Bio, and placed the Tracker Fund sixth on almost entirely sell-side volume — HK$604.7 million sold against HK$101,360 bought. Semiconductor, AI and index-ETF lines dominate both lists. That is a description of what traded, not an explanation of why, and the difference matters.
The rules behind the numbers
Two things about this channel are routinely misread. The first is who stands on each side. Southbound is open to members of the Shanghai and Shenzhen exchanges and to institutional and individual investors who satisfy eligibility criteria in the mainland. Northbound is open to Hong Kong exchange participants and their clients, and mainland investors are barred from buying through it at all — HKEX's Stock Connect FAQ, in a version dated 6 July this year, is explicit that they may sell what they already hold but may not initiate purchases. The two directions are not mirror images and should not be read as one number with a sign on it.
The second is the quota. Each of Shanghai Connect and Shenzhen Connect carries a daily quota of RMB 42 billion, applied on a net-buy basis, so a heavy selling morning returns capacity rather than consuming it. The aggregate quota was abolished in 2016. A session's gross turnover can be enormous without the quota being touched at all, which is why the net matters to the plumbing and the gross matters to the market.
Eligibility is narrower than "Hong Kong stocks". The FAQ sets out one test: a constituent of the Hang Seng Composite SmallCap Index with a market capitalisation of at least HK$5 billion is eligible for southbound trading, and a constituent without a matching A-share listing is designated sell-only once its average month-end capitalisation over the prior twelve months falls below HK$4 billion. One further trap for readers of the tables: southbound turnover is published in Hong Kong dollars and northbound in renminbi. HKEX labels them that way in its monthly summary, and the two series are not comparable until converted.
What to watch
The next fortnight of files will settle a narrower question than the index invites. August's HK$2.25 trillion of gross turnover has given way to a September running 19.6 per cent below that pace, while the net — positive in eleven sessions out of twelve, and small every time — has kept the same shape throughout. What would change the character of the flow is not one session's turn but a run of them, and the signals are dull ones: consecutive net buys or net sells, the buy-to-sell ratio drifting one way, the net-buy balance against the RMB 42 billion ceiling. A reader looking for a change of mind in mainland money should watch the sequence, not a Friday.
