By Jennifer Parker*
In the wake of the US strikes on Iran, focus has shifted to how Tehran will respond. Its options range from direct attacks on US bases to exerting pressure on maritime trade through the Strait of Hormuz. The Iranian parliament’s reported vote to blockade the strait offers a possible clue. Can Iran realistically close this chokepoint, and what would that mean for Australia?
Wedged between Iran and Oman, the Strait of Hormuz is the only maritime gateway to the oil-rich Persian Gulf, carrying about a quarter of the world’s crude exports. Iran’s control of the northern shore has long fuelled fears it could shut the strait in retaliation to an attack. The threat is hardly notional: Tehran has used shipping harassment for leverage before, including during the 1980s “Tanker Wars” with Iraq.
After Trump quit the Joint Comprehensive Plan of Action nuclear deal in 2018 and launched its “maximum-pressure” campaign, Tehran again turned to commercial shipping. In May 2019, four tankers were attacked with limpet mines in the Gulf of Oman, almost certainly by Iran. Two months later, Iran’s Revolutionary Guards (IRGC) seized the UK-flagged tanker Stena Impero in the Strait of Hormuz and briefly detained the Liberian-flagged Mesdar. Those incidents opened a two-year stretch of Iranian harassment of civilian and naval traffic in the world’s busiest oil chokepoint.
After Iran’s 2019 attacks on commercial shipping, Washington set up the International Maritime Security Construct to protect shipping, with Australia among its founding members. The strait’s security is critical to Australia’s economy, which is why Canberra sent a warship and personnel, including me, to help keep it open.
Long before Iran’s April 2024 missile barrage on Israel, the two rivals were already skirmishing at sea. In March 2021, an Israeli-owned freighter was hit by what was almost certainly an Iranian missile in the Arabian Sea. A month later, limpet mines widely blamed on Israel crippled the Iranian-flagged MV Saviz in the Red Sea, a vessel believed to serve as an IRGC forward base. These incidents show how the proxy war spills into maritime space and how Tehran uses strikes on merchant shipping for strategic signalling.
Iran views its grip on the Strait of Hormuz as its trump card and has repeatedly harassed and attacked commercial and military vessels transiting the strait to make a political point. It is therefore no surprise that the Iranian parliament has reportedly approved a motion to blockade the waterway. Whether Tehran can, or will, carry it out is another question.
Naval blockades are back in vogue: Russia’s bid to choke Ukraine’s grain exports in the Black Sea, Houthi claims of blockading the Red Sea to Israel-linked ships, and fears that Beijing might apply a naval blockade to ring-fence Taiwan all show how coercion at sea is reshaping security debates.
Naval blockades are lawful under the law of armed conflict, but only if they meet strict tests: they must be formally declared and notified, enforced impartially and effectively, and limited to stopping enemy commerce or contraband. Crucially, a blockade cannot starve civilian populations or seal off neutral ports.
An Iranian blockade of the Strait of Hormuz would choke access to neutral gulf ports without targeting contraband or enemy vessels – neither the United States nor Israel is based inside the gulf, Bahrain’s US naval facility notwithstanding. Because it would indiscriminately impede neutral trade, such a move would fall outside the legal limits of naval blockade, despite the Iranian Parliament’s reported approval.
Even if an outright blockade breached international law, Tehran could still attempt to close the strait. Its conventional navy is ageing and limited, but the Islamic Revolutionary Guard Corps Navy fields swarms of fast-attack craft, anti-ship missiles, drones and mines – an asymmetric mix designed for narrow-water choke-points. Iran would not even have to act decisively: a simple claim that it had laid mines could divert commercial traffic until US-led forces proved the channel clear, a process that could take weeks.
Despite years of rhetoric, Tehran is unlikely to close the strait. Its strategic partnership with Beijing is a brake: China is the world’s largest crude importer, and roughly half of its oil arrives via gulf producers transiting the strait. Crippling that flow would undercut a key supporter and damage Iran’s own diplomatic gains with Beijing.
China is hardly alone in its dependence on Strait of Hormuz oil. A closure would jolt the global economy and bite Australia in particular. Despite the 8000-kilometre distance, most of Australia’s crude arrives via this chokepoint, and the nation imports about 91 per cent of its fuel.
Most of Australia’s petrol, diesel and jet fuel arrives as finished product from refineries in Singapore, South Korea and Japan, but those refineries source much of their crude from the Middle East via the Strait of Hormuz. Any prolonged disruption there would therefore ripple straight down Australia’s supply chain. The risk is magnified by Canberra’s chronic shortfall against the International Energy Agency’s 90-day stockpile obligation: as of March 2025, Australia held barely 56 days of fuel in reserve.
Australia’s 1990-2020 naval deployment to the Middle East was never mere alliance diplomacy; it safeguarded the long sea lines that bring fuel to Australia. That dependence remains, yet decades of under-investment leave the Royal Australian Navy with only 10 surface combatants until the 2030s – several in refit – hardly able to assist in breaking an Iranian blockade today if Washington came calling.
Despite the Iranian parliament’s vote to “blockade” the Strait of Hormuz, Tehran is unlikely to follow through. Shutting the waterway would penalise neutral gulf ports and anger China – its most important economic partner and a veto-wielding UN Security Council member. Even so, experience shows any escalation will spill into the maritime domain. For Australia, the message is clear: rebuild strategic fuel stocks to meet International Energy Agency obligations and strengthen supply-chain resilience; and, as an island nation reliant on vulnerable sea-lines of communication, invest now in a navy capable of keeping them open.
This article first appeared in The Sydney Morning Herald.
*Jennifer Parker is Adjunct Fellow, Naval Studies at UNSW Canberra, and Expert Associate, National Security College, Australian National University
