Why stocks can move after 4pm on the ASX
During results season in February this year, the ASX-listed biotech behemoth CSL announced the resignation of its chief executive.
It released the news at 4:05pm, reportedly believing the market was closed.
But while 'normal trading', where buy and sell orders are matched, had ceased, the market was not closed.
At 4:05pm, there are still phases in the trading day that can impact on a stock's price - as CSL discovered when its shares dropped 5%.
Why stocks can still move after 4pm
Investors get perplexed about why stock prices change after the end of normal trading. Fair enough.
It's a truth universally acknowledged that the ASX market is open from 10:00am to 4:00pm, isn't it?
However, like a good Regency novel, the truth is more complicated.
The many different phases in a trading day allow for different actions to occur.
Let's look at those phases on the ASX, where the bulk of cash equities trading, including shares and exchange traded funds, takes place in Australia.
Other local markets, NSX and Cboe Australia, have similar trading day cycles.
Pre-open
Buy and sell orders are entered into the system, ASX Trade, by brokers on behalf of their clients and queued according to price-time priority, see Explainer 2.
Orders can be amended or cancelled but won't be matched or executed until the market opens.
Opening single price auction
Opening auction, see Explainer 3, takes place where buy and sell orders are matched to become trades.
Orders can be cancelled but new orders and amendments are not accepted.
This is followed by a levelling or buffering period to enable orderly processing by broker systems.
Normal trading
Opening prices are calculated at the start of this phase, after which ASX Trade matches buy and sell orders in price-time priority on a continuous basis until 4pm.
What happens between 4pm and the true close
All stocks open for trading at the same time, replacing the arrangement preceding June 2025 whereby stocks opened on a staggered alphabetical basis over approximately nine minutes.
System robustness now allows large, concentrated volumes to be handled smoothly and efficiently.
Pre-closing single price auction
Continuous matching of orders ceases.
Brokers can enter, alter and cancel orders in preparation for the market close.
Closing single price auction (CSPA)
ASX Trade calculates a consensus closing price for each stock.
Post-close
New orders can be entered, existing orders amended and fresh orders executed at closing prices set during the CSPA.
Cancellations are also permitted.
Why late announcements can still move prices
Since June 2025, a listed company that releases a price sensitive announcement, such as the resignation of a chief executive, between 4:00pm and 4:10pm has its own closing auction in this phase, instead of missing the closing auction window.
This gives investors the opportunity to react to the news, adjust their orders and access the additional liquidity in this phase.
Adjust and Adjust ON
A tidying up period where orders may be cancelled and amended.
But new orders cannot be entered nor trades executed.
Then, orders that have expired or are too far from the market will be purged from the system, followed by a system maintenance adjustment state.
During the Close phase, trading messages cannot be entered or amended, and no matching or auctions occur.
The system unavailable phase enables securities to be added for the next trading day.
Why the ASX doesn't pause at lunch
One phase we don't have in Australia is a lunchtime trading pause, which is common among Asian exchanges including Tokyo, Hong Kong and Shanghai.
Originally serving an operational purpose, its practice today is largely cultural.
With the ever-faster pace of markets, and life generally, perhaps the time for a forced stop in the middle of our day has come.
Fairness, orderliness and efficiency underpin many of the different phases of a trading day.
Knowing what you can do and when is critical to being an informed and effective investor.
Why understanding market phases matters
Explainer 1
Randomised time windows are used as an orderly way to manage high-volume concentrations.
The random timing reduces excess volatility and the opportunity for 'gaming' or manipulating the system, and improves price discovery.
Explainer 2
Price-time priority is a standard exchange fairness and efficiency mechanism, which prioritises orders with the best price (either highest bid or lowest offer) and then, if there are multiple orders at the same price, according to the time received.
Explainer 3
Auctions allow investors to trade at a single point in time, bringing together the widest range of counterparties, delivering transparent and effective price formation.
An auction algorithm is used to determine a fair matching price at the start and the end of the trading day.
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