Reading the ASX Opening Auctioneric

This is part three of the ASX order book series. After protection, the second idea that dominated the intraday journals I read was the opening auction. Traders called it "the match." For some of them it was close to their entire edge: a large share of their profit came from the first seconds to minutes after each stock opened.

There's a good reason for that. The ASX open is the one moment each day where every participant's overnight information, every reaction to news, and every institutional order collides at a single price. It's also the one moment where the exchange shows you, in advance, roughly where that price will be.

How the ASX open works

Before the open, the market is in pre-open. Orders can be entered, amended and cancelled, but nothing trades. The exchange continuously calculates the price that would maximise the volume traded if the auction ran right now. That price is the indicative opening price (often shown as IEP or "indicative" in trading platforms).

At the open, a single price auction runs. All orders that can trade at the auction price trade at that one price. Whatever is left over becomes the opening order book for normal continuous trading.

Historically the ASX has opened stocks in staggered groups, by the first character of the ticker, a couple of minutes apart, with a small random element in the exact time of each group. That staggering mattered to traders because it let them focus on one group at a time and plan their orders per group. Auction arrangements do change, so always check the current ASX operating rules for exact timings rather than relying on a blog post.

Two numbers matter most during pre-open:

Surplus: the most underrated number on the screen

If the auction would clear at 1.20 and there are more buy orders at 1.20 or above than sell orders at 1.20 or below, the excess is a bid surplus. Those buyers didn't get filled in the auction. They are still sitting there, at the opening price, the instant continuous trading starts.

Conversely, an offer surplus means unfilled sellers are waiting at the open price.

The traders I read were blunt about this. For a long off the open, they wanted to see a bid surplus: unfilled buyers at the opening price, which acts as protection immediately underneath them. A long that opens with an offer surplus is starting life with sellers sitting right on top of it.

These are tendencies, not laws. But they're the starting point for nearly every match setup I saw described.

The last 30 seconds

Pre-open orders are free to cancel, so a lot of what you see early in pre-open is positioning, testing and noise. What matters is the picture just before the auction runs.

The journals described a specific pattern repeatedly: a long thesis that looked fine for most of pre-open, then fresh sellers arriving in the final seconds, the bid side thinning, or the surplus flipping sides. Experienced traders treated that as decisive. They pulled their order and walked away, regardless of how good the news looked. Beginners hesitated, got filled, and owned the trade at the worst moment.

Things to watch in the last stretch before the auction:

• The surplus flipping from bid to offer, or the reverse.
• The indicative price drifting away from your planned entry.
• Large orders appearing late on the side that would hurt you.
• Large orders disappearing from the side that was protecting you.
• The overnight lead moving during pre-open: futures or a commodity sliding against your trade.

Is the open mispriced?

At its core the match trade is a judgement about whether the auction price is too high or too low. The traders used a short list of reference points to decide.

Versus the overnight lead. If US markets, futures, a relevant commodity or a stock's foreign listing point one way, does the indicative gap agree? (This gets a full post later in the series.)

Versus sector peers. If every gold miner is indicated up 2% and one is indicated up 6% with no news of its own, that one is the outlier.

Versus the cause of the gap. A gap driven by genuine news is different from a gap driven by one large seller dumping into the auction. (Also its own post.)

Versus the book. Is there protection behind the trade? Is the path ahead clear? Is the matched volume large enough that you can actually get in and out?

Trading the match versus trading the open

There are two broad ways to act on an auction read.

Participate in the auction. Place your order in pre-open so it trades at the opening price. You get the auction price and you're in before continuous trading starts. The risk is that your order itself can move the indicative price in a thin stock, and you can't control your exact fill as the auction price moves around.

Trade the first seconds after the open. Wait for the auction, then act on what the opening book looks like. You see the result before committing, but the best part of the move may happen in the first few seconds.

Either way, the common thread in the journals was speed of the exit. A match trade is a bet that the open is mispriced and will correct quickly. Many winning match trades described were measured in seconds: a few ticks, very soon after the open. If the move didn't come, the trade was scratched.

Common mistakes

These came up again and again, usually in the early months of a journal:

• Amending orders as the stock opens, and getting filled at a much worse price than planned.
• Forgetting an order in pre-open and getting filled on a stock you'd stopped watching.
• Shorting without borrow arranged before the auction.
• Trying to trade too many auctions at once, then executing all of them badly.
• Chasing after missing the match, buying the stock a few seconds later at a worse price with no protection.
• Ignoring matched volume, taking a size that the opening liquidity couldn't absorb.

A pre-open routine

A compressed version of the routine I saw across the more consistent traders:

That's the open. The next post looks at the single most useful question to ask about any gap: why did it gap?