Portfolio concentration risk: when a winning position becomes a dangerous one

How concentration risk develops

Concentration risk rarely starts as a deliberate choice. It develops in two common ways:

The winner that keeps winning. You buy $10,000 of Palantir, Nvidia, or a high-conviction ASX growth stock. It doubles, then doubles again. Your other positions grow modestly. Suddenly a position that was 8% of your portfolio is 35% — without you adding a single dollar.

The employer share plan. You receive shares or options in your employer over several years. The company performs well. You now hold $250,000 in a single company — the same company that pays your salary, owns your career, and could make you redundant in the same event that crushes the stock. This is dual concentration: financial and employment exposure in one entity.

What the numbers actually say

Consider a portfolio where one position reaches 40% of the total. What does a bad quarter look like?

Now compare to a well-diversified portfolio where no single stock exceeds 10%:

The same 30% single-stock drawdown costs the concentrated portfolio four times more. And the concentrated investor now faces a psychological trap: the temptation to hold through the drawdown to "get back to even" rather than reassessing the thesis objectively.

Sector concentration: the hidden version

Single-stock concentration is obvious. Sector concentration is harder to see. A portfolio of 15 different stocks across Palantir, Nvidia, Microsoft, Salesforce, Atlassian, Xero, WiseTech, and several smaller tech names might look diversified. In reality, it's a single-sector bet. When AI sentiment shifts, rate expectations change, or tech multiples compress, every position moves together.

Correlation is the enemy of apparent diversification. Stocks in the same sector, with the same macro sensitivities, or that serve the same customer base tend to sell off together — exactly when you need your "other positions" to hold the portfolio together.

Concentration type	What to measure	Warning threshold
Single stock	Position as % of total portfolio	>15% in any one stock
Sector	Sector weight as % of portfolio	>40% in any one sector
Geography	Country or market exposure	>80% in one country (for active portfolios)
Currency	USD / AUD / other FX exposure	Unhedged FX as significant % of total
Employment	Employer stock as % of total wealth	Any meaningful % — salary already concentrates risk

The tax trap: why investors hold too long

One of the most common reasons Australian investors sit on over-concentrated positions is simple: CGT. A position that has tripled carries a significant embedded gain. Selling feels painful because you're handing 23.5% (at 45% marginal rate with the 50% discount) of your gain to the ATO.

This is rational tax management taken too far. If a position is genuinely too large and poses real portfolio risk, the CGT tail is wagging the investment dog. The question isn't "how much tax will I pay?" — it's "how much would a 40% drawdown in this position cost me compared to the tax I'm trying to avoid?"

In this case, paying the $23,500 tax to rebalance protects against a potential $60,000 loss — a trade-off many investors avoid because the tax is certain and the drawdown is only probable. But probability-weighted, if there's even a 50% chance of a 40% correction, the expected cost of inaction ($30,000) exceeds the certain cost of rebalancing ($23,500).

How to decide what's "too concentrated"

There's no universally correct answer — it depends on your conviction in the position, your overall wealth level, your income stability, and how much portfolio volatility you can psychologically and financially withstand. But some useful frameworks:

The sleep test

If the position fell 40% tomorrow, would you sleep? Would you be able to hold it through the drawdown without panic-selling at the bottom? If not, it's too large — not because the position is wrong, but because the size is beyond your psychological capacity to manage.

The "what if I'm wrong?" test

For every concentrated position, ask: what if my thesis is completely wrong? Not partially wrong — completely wrong. What's the impact on your total portfolio? If being wrong about one company would materially set back your financial goals, the position is too large regardless of how confident you are.

The Kelly Criterion

The Kelly Criterion is a formula from probability theory used by professional gamblers and traders to size positions: bet size = (win probability × odds − loss probability) / odds. It's mathematically optimal for long-term wealth growth — and it almost always produces smaller position sizes than intuition suggests. Most investors overestimate win probability and underestimate loss magnitude.

Strategies for reducing concentration

Staged trimming

Rather than selling an entire position, trim it back by 20–30% to reduce exposure while maintaining upside. Set a target weight (e.g. 10% of portfolio) and sell enough to reach it. Repeat if the position grows back above the threshold.

Use future contributions to rebalance

Instead of selling, direct all new contributions into underweight positions until the overall allocation returns to balance. This avoids triggering CGT while gradually correcting the imbalance. It works best when the imbalance isn't extreme and you have ongoing investment capacity.

Protective puts (for sophisticated investors)

Options can provide downside protection on a concentrated position without triggering a CGT event. Buying put options on a large position caps your downside loss in exchange for an upfront premium. This is a more complex strategy that requires options access and understanding — but it separates the tax decision from the risk management decision.

Accept the CGT and move on

Sometimes the right answer is simply to pay the tax, rebalance properly, and invest the after-tax proceeds into a well-diversified portfolio. The compounding value of a properly balanced portfolio over 10–20 years typically dwarfs the one-time tax cost of rebalancing.