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The Hidden Currency Gain: CGT on US Shares Explained

Your broker charged you a foreign exchange fee you didn’t see. But there’s a bigger hidden cost in buying US shares — and this one shows up at tax time.

Wall St. Down Under

Wall St. Down Under

— 5 min read
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Last week we broke down the FX fee your broker quietly clips on every US share trade — the spread between the wholesale rate and what you actually pay. Most Australian investors lose 0.5% to 1.5% per transaction in invisible exchange costs.

That is a transactional cost. Today we are talking about a structural one, and it is significantly larger.

When Australian investors buy US shares there are two sources of gain or loss at tax time: the movement in the share price, and the movement in the AUD/USD exchange rate. Most investors know about the first. Fewer fully understand how the second is calculated — and it catches a lot of people when their tax return comes in higher than expected.

How the ATO Calculates Your Capital Gain

The ATO requires Australian tax residents to calculate capital gains in Australian dollars. Always. The share may be priced in USD, but the gain or loss is denominated in AUD.

Here is how the cost base works:

Cost base = Number of shares × USD purchase price × AUD/USD exchange rate at date of purchase

Proceeds = Number of shares × USD sale price × AUD/USD exchange rate at date of sale

Capital gain = Proceeds − Cost base

Notice what is embedded in that formula: the exchange rate at both ends. If the USD strengthens against the AUD between your purchase and your sale — even if the share price in USD is completely flat — you will have a taxable capital gain in AUD.

This is not a loophole. This is not avoidable. It is the structural reality of investing in a foreign-denominated asset as an Australian tax resident.

Your broker statement will usually show the AUD amount it used for the conversion. Keep that record. If you need an independent reference, the Reserve Bank of Australia’s historical daily rates are the standard source accepted by the ATO. Incidental costs such as brokerage are also converted at the relevant rates and form part of the cost base or proceeds.

A Worked Example

Let’s make this concrete.

Purchase:

  • You buy 10 shares of a US company at US$100 each
  • The AUD/USD rate on the day you buy: 0.65 (1 AUD buys US$0.65)
  • USD cost: US$1,000
  • AUD cost base: US$1,000 ÷ 0.65 = AUD $1,538

18 months later, you sell:

  • The share price is unchanged — still US$100 per share
  • But the AUD/USD rate has moved to 0.60 (the AUD has weakened; 1 AUD buys US$0.60)
  • USD proceeds: US$1,000
  • AUD proceeds: US$1,000 ÷ 0.60 = AUD $1,667

Your capital gain:
AUD $1,667 − AUD $1,538 = AUD $129

The share price didn’t move a single cent in USD. But you made a taxable capital gain of $129 in AUD purely because the exchange rate moved.

Now apply the 50% CGT discount — because you held for at least 12 months, only half the gain is added to your taxable income:

Taxable gain: AUD $64.50

At a marginal tax rate of 39% (including Medicare levy), that’s approximately AUD $25 in tax on a position that went nowhere in USD terms.

The reverse is also true. If the AUD strengthens over the same period, a flat USD share price can produce an AUD capital loss.

The 12-Month CGT Discount: Why It Matters

The ATO provides a 50% discount on capital gains for assets held for at least 12 months. This is one of the most valuable provisions in Australian tax law, and it applies equally to foreign shares.

The practical implication: there is a meaningful tax difference between selling a position at 11 months versus 13 months. At a 39% marginal rate:

  • Hold less than 12 months: pay 39% on the full gain
  • Hold at least 12 months: pay 39% on half the gain — effectively a 19.5% rate

On a $5,000 gain, that’s the difference between $1,950 in tax and $975. The one-year mark is not a rule to be vague about.

Note for Australian investors currently sitting on a loss: If you’re in a losing position on a US share, the same FX mechanics work in reverse — a weakening USD (or strengthening AUD) can increase your AUD-denominated loss, which may offset other capital gains in your portfolio. Worth factoring in before you decide whether to crystallise that loss.

What the FX Fee Article Showed You — and How This Connects

Last week’s piece on broker FX fees was about the hidden cost at the point of transaction. This week is about the hidden variable that persists for the entire duration of your investment.

The FX fee is a fixed percentage taken once. The currency gain or loss accumulates every day you hold the position, invisibly, and only becomes visible when you sell and do your tax return.

Together they make the same point: when Australian investors buy US shares, currency is not a detail. It is a core investment variable — as important as the P/E ratio, the earnings trajectory, or the dividend yield. It affects your entry price, your holding cost, your exit proceeds, and your tax liability.

The investors who understand this are the ones who do not get a nasty tax surprise in July. The ones who do not understand it often think they have had a modest year in the market, and then discover at tax time that the AUD’s movement created a taxable gain they had not accounted for.

A Practical Checklist for Australian US Share Investors

✅ Record the exchange rate on every purchase date — your broker statement should include it, but keep your own record.

✅ Record the exchange rate on every sale date — same principle.

✅ Track your 12-month holding dates — the CGT discount is too valuable to miss by 30 days.

✅ Consider FX when evaluating position performance — a 10% gain in USD might be a 6% gain in AUD if the Australian dollar has strengthened, or a 14% gain if it has weakened. Know which one you actually have.

✅ Talk to your accountant before EOFY — if you have unrealised gains and losses across multiple positions, proactive harvesting before June 30 can meaningfully reduce your taxable gain.

Data Sources:

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Disclaimer: Wall St. Down Under is an independent financial newsletter for informational and educational purposes only. Nothing published here constitutes financial advice, a recommendation to buy or sell any security, or a solicitation of any investment decision. Always do your own research and consult a licensed financial adviser before making investment decisions. Australian investors should consider their own financial situation, objectives, and risk tolerance. Past performance is not indicative of future results.

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