When it comes to long-term investing, the real danger isn't just picking a loser — it is letting your winners get too big.
We all love it when an ASX stock surges and transforms into a cornerstone of our portfolio. But there comes a point where even the strongest performers can expose you to risk, simply by becoming too large a piece of the overall pie.
That's where portfolio rebalancing comes in — a simple, powerful tool that helps investors stay diversified, manage downside risk, and keep their strategy on track.
The risk of concentration
Let's say you bought into a company like CSL Ltd (ASX: CSL), Pro Medicus Ltd (ASX: PME), or Xero Ltd (ASX: XRO) early on. Great call. But fast forward a good number of years and it has now grown to make up 30%, 40%, or even more of your portfolio's value.
If the company continues performing, no problem. But what if it stumbles — or posts a surprise earnings miss?
A single ASX stock with that much portfolio weighting can suddenly sink your portfolio's performance, and potentially your confidence too. No one expects their star performer to disappoint… until it does.
Rebalancing gives you the chance to get ahead of that scenario — before market reality forces your hand.
Rebalancing option 1: Add to other positions
If you are reluctant to sell a high-performing ASX stock (and many investors are), one way to reduce its weight is by simply adding funds to your other positions. This is my preferred option.
By growing the size of the rest of your portfolio, your dominant stock becomes a smaller percentage over time — allowing you to let your winners run while still regaining diversification.
This is a particularly useful strategy if you're adding fresh capital regularly, such as monthly contributions. It is less emotionally taxing than selling, and it respects the power of compounding.
Rebalancing option 2: Trim and reallocate
Sometimes, though, you may simply not have the required capital to effectively rebalance your portfolio. On this occasion, rebalancing does mean trimming.
This doesn't mean bailing on your best idea. It just means taking some profits, reducing the weight back to a more balanced level (maybe 10% to 15%), and then reinvesting into areas of your portfolio that have more room to run — whether that's a promising growth stock, a diversified ASX ETF, or a sector that's temporarily out of favour.
The main thing to understand is that it is not about punishing success. It is about protecting it.
