Concurrent Delay: Who Bears the Cost When Two Delays Collide

"The delays were concurrent" is one of the most common — and most commonly misused — assertions in Australian delay disputes. Respondents reach for it to defeat prolongation claims; contractors fear it as the argument that turns a strong EOT into a costs-free one. The reality is narrower and more interesting: genuine concurrency is rare, the Australian case law is surprisingly thin, and the outcome usually turns less on doctrine than on whose records can untangle the overlap.

What concurrency actually is — and isn't

The SCL Delay and Disruption Protocol (2nd edition) draws the essential distinction. True concurrent delay is "the occurrence of two or more delay events at the same time, one an Employer Risk Event, the other a Contractor Risk Event, and the effects of which are felt at the same time" — and, critically, each must be an effective cause of delay to completion, meaning both must independently hit the critical path. The Protocol's own assessment: "True concurrent delay will be a rare occurrence."

What practitioners usually mean is the more common situation of sequential delays whose effects overlap — the contractor was already running late on its own account when the principal's variation landed, or vice versa. The legal treatment of the two situations differs, and conflating them is where most concurrency arguments go wrong.

The Australian position: thin authority, common-sense causation

There is no High Court or intermediate appellate decision squarely settling concurrent delay entitlement in Australia. What exists points in a consistent direction:

• First in time matters. In Australian Development Corporation Pty Ltd v White Constructions (ACT) Pty Ltd (1996) 12 BCL 317, no extension was available for a qualifying event where the contractor was already critically delayed by its own risk event that continued beyond the qualifying event's effects. A delay event must actually drive the critical path to count.
• Causation is decided on the facts, not the model. White Constructions Pty Ltd v PBS Holdings Pty Ltd [2019] NSWSC 1166 applied the common-law common-sense approach from March v E & MH Stramare Pty Ltd (1991) 171 CLR 506 — the same approach that governs untangling overlapping causes.
• The orthodox money outcome, absent contrary contract terms: where a principal-risk and a contractor-risk delay are genuinely concurrent, the contractor generally has the better of the time argument (consistent with the English Henry Boot v Malmaison approach and the Protocol's Core Principle 10) but cannot recover prolongation costs for the concurrent period — because it cannot show it would not have incurred those time-related costs anyway. Time, not money. The Protocol's compensation principle makes the same point from the records end: the contractor recovers prolongation compensation only if it can separate the costs caused by the employer delay from those caused by its own.

The contract can — and increasingly does — decide it

Standard forms diverge. AS 4000 (cl 34.4) takes a distinctive path: where qualifying and non-qualifying causes of delay overlap, the superintendent apportions the resulting delay according to the respective causes' contribution — more contractor-friendly than AS 2124's treatment, though the apportionment metric is famously undefined. Delay damages under cl 34.9 still require a compensable cause, so the time/money split survives apportionment.

Amended contracts increasingly go further, importing English-style exclusion clauses: no EOT to the extent the delay is concurrent with contractor-caused delay. In North Midland Building Ltd v Cyden Homes Ltd [2018] EWCA Civ 1744 the English Court of Appeal upheld exactly such a clause, holding the prevention principle is not an overriding rule of law and yields to express allocation. Australian courts have not ruled on the precise drafting, but the reasoning in Spiers Earthworks Pty Ltd v Landtec Projects Corporation Pty Ltd (No 2) [2012] WASCA 53 — the prevention principle operates through implied terms the parties can modify — and the strict enforcement of risk allocation in cases like CMA Assets point the same way. If your contract has a concurrency exclusion, assume it works.

The records decide what the doctrine can't

Concurrency disputes are won and lost on the ability to place each delay on the critical path, in time, with evidence: contemporaneous programme updates showing what was actually driving completion in each window; daily records showing which crews were where, idle or working, and why; cost coding that can separate the time-related costs of the employer delay from the contractor's own. Without that, "concurrent delay" collapses into competing assertions — and tribunals applying common-sense causation will default to whichever party's records tell a coherent story.

What this means in practice

1. Interrogate "concurrency" claims before conceding them. Most alleged concurrency is sequential delay with overlapping effects, or a non-critical contractor delay running alongside a critical principal delay. Demand the critical-path demonstration for both limbs — true concurrency requires each event to be an effective cause of delay to completion.
2. Contractors: protect the time claim, fight for the money separately. The pragmatic position in genuine concurrency is to secure the EOT (shielding against LDs) and then build the prolongation claim only for windows where the employer delay was the sole effective cause — with cost records segregated accordingly.
3. Read your concurrency clause now. AS 4000 apportions; AS 2124 doesn't; amended forms may exclude entirely. Each demands different claim strategy and different records, and the North Midland line means exclusions will likely hold.
4. Respondents and superintendents: apportionment needs a method. Under AS 4000 cl 34.4 you must apportion "according to the respective causes' contribution" — an assessment that will be scrutinised. Document the analysis; an unexplained 50/50 split invites challenge.
5. Build the untangling capability during delivery. Updated programmes, statused honestly each month; delay event registers identifying cause ownership in real time; cost codes that can isolate prolongation by window. Concurrency is the dispute that punishes generic record-keeping hardest.

Key takeaways

• True concurrent delay — both events independently driving the critical path at the same time — is rare; most "concurrency" is overlapping effects of sequential delays.
• Australia has no apex authority; courts apply common-sense causation, and first-in-time critical delay matters (ADC v White Constructions; White Constructions v PBS).
• The orthodox default is time-not-money: an EOT may be available, but prolongation costs fail where they cannot be separated from contractor-caused delay.
• The contract can reallocate all of this — AS 4000 apportions (cl 34.4); modern exclusion clauses follow North Midland and should be assumed enforceable.
• Records that can place each delay on the critical path, window by window, decide these disputes.

This article is general information only and is not legal advice. For advice on a specific contract or dispute, seek legal counsel or contact Sumit Consulting for commercial and claims advisory support.