Direct answer
The spread in AUD/USD is mainly affected by (1) liquidity, (2) volatility, (3) where and how orders are executed, and (4) provider policies and cost structure. These factors change how much it costs and how quickly a market participant can quote both a buy and a sell price.
Mechanism and definition
A currency pair spread is the difference between the quoted buy price (bid) and the quoted sell price (ask). If you trade immediately at the market quote, the spread becomes part of the immediate transaction cost.
Four groups of drivers are usually responsible for changes in AUD/USD spreads:
Liquidity
Liquidity means how easily market participants can buy or sell AUD/USD without moving the price too much. When there are more active buyers and sellers, quoting can be tighter, because the market maker or liquidity provider can offset risk more easily. When liquidity drops, the quoted buy and sell prices tend to move farther apart to compensate for the higher chance of being stuck with inventory.
Volatility
Volatility is how quickly prices move. Higher volatility increases the risk that a quote becomes outdated before it is filled. To manage that risk, providers typically widen the spread so the potential loss from adverse price movement is reduced.
Execution venue and trade mechanics
Even when the displayed spread looks similar, the effective cost can differ because execution depends on order routing and market access. For example, whether an order is filled from available quotes, partially filled across venues, or delayed by matching/processing can change the realized price relative to the last quoted bid/ask. In other words, the “spread you see” may not fully represent the “price you actually get,” especially for larger orders or during busy periods.
Provider policy and cost structure
Providers can use different ways to cover operating costs: some charge commission plus a narrower spread; others embed costs more into the spread. The net result is that the spread level you observe, and the total all-in cost, can vary with the provider’s pricing model, hedging approach, and execution rules (for example, how they handle order slippage and liquidity provision).
Evidence or example (with clear assumptions)
Assume two moments of trading in AUD/USD:
-
Moment A (higher liquidity, lower volatility): Many participants are quoting both sides actively, and prices change more slowly. Under these conditions, maintaining tight bid/ask quotes is less risky, so spreads tend to be smaller.
-
Moment B (lower liquidity, higher volatility): Fewer participants quote continuously, and price swings are faster. Quotes can be hit before conditions change, so providers widen spreads to reduce the risk of holding adverse positions.
If a provider uses commission-based pricing, the quoted spread may appear narrower, but commission adds to the all-in cost. If the provider uses spread-only pricing, the spread itself tends to reflect costs more directly. The key point is that you should treat “spread” as one component of trading cost, not the whole cost.
Limitations and risks
- No real-time guarantees: Spreads are dynamic. Even the same general conditions (for example, a known busy time) do not guarantee a particular spread.
- Realized cost may differ from quotes: The quote you see can differ from what you receive due to execution timing, partial fills, and order size.
- Provider rules can change outcomes: Different execution rules and pricing models can alter the effective cost without changing the underlying market.
- Failure mode—confusing correlation with causation: Liquidity and volatility often move together. A wider spread may reflect both, or other market stress, so attributing the cause to only one factor can be misleading.
How to verify independently (next question)
To verify what is affecting AUD/USD spreads for a specific situation, compare the conditions around the time you observed the spread change:
- Look for changes in trading activity (liquidity) and the speed of price movements (volatility).
- Consider execution factors: market vs limit behavior, and whether orders were filled immediately or after delay.
- Check your provider’s cost model: whether it is primarily commission-based, spread-based, or a mix, because “spread” alone may not reflect all-in cost.