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Tank Traps in Dee Why Lagoon

By Alec Smart


'Dumble Stops' rotting in Dee Why Lagoon, left over from WWII. Photo: Alec Smart


At the southern end of Dee Why Lagoon, hidden behind bullrushes and almost submerged by incoming tides, lies a dual row of rotting timber poles. Resembling a ruined jetty, these forlorn stumps are among the last of Australia’s World War 2 sea defences still in place.


This small, forgotten cluster of approximately 22 timber staves were, from June 1942 – Sept 1943 (when Prime Minister John Curtin declared that ‘the danger of invasion has passed’) among the front line of Sydney’s deterrents to a feared Japanese naval incursion.

Timber bollards (derived from the Norman-French word boulard, meaning ‘post’), known as ‘Dumble tank stops’, were driven into the beds of seaside lagoons, tidal lakes and creeks.


In addition to bollards, shoreline defences included ‘concertina’ rolls of barbed wire, iron stakes, ‘hurdles’ consisting of interlocked steel scaffolding, and lines of pyramid-shaped tetrahedrons made of reinforced concrete.

The latter pyramids (some flat-topped) were typically installed in rows near beaches or on the tideline to halt the advance of landing craft, tanks and mechanised machinery and were known by the amusing term ‘dragon’s teeth’.


A single dragon’s tooth still sits among the mangroves at Bayview, opposite 1961 Pittwater Rd, the lonely reminder of a bygone era, regularly submerged by incoming tide.


It’s possible Dumble stops were named after Lieutenant-Colonel Wilfred Chatterton Dumble, former General Manager of London Omnibus Company. W.C. Dumble was one of the key members of the Admiral Landships Committee in Britain that oversaw the construction and military engagement of the world’s first military tanks.


In co-developing the tank, in all likelihood Dumble also realised that their progress could be hindered by clusters of bollards, and maybe that’s how these timber staves inherited his name.


The pyramid-shaped dragon’s teeth had the benefit of an armour-piercing sharp tip that would kinetically puncture the metal underside of a tank if the vehicle attempted to roll over the top.


However, the main tactic of Dumble stops and dragon’s teeth was not to pierce tanks but impede and channel them into zones where they could more easily be damaged/destroyed by heavy armaments from planes or ground defence.


Dragon's Teeth - how Dee Why Beach probably looked during WWII. Graphic: Alec Smart


After several submarine attacks on merchant shipping off the coast of NSW in 1942, and Japanese air raids on Darwin (19 Feb 1942) and Broome (3 March 1942) causing hundreds of fatalities, Sydney was on high alert for a suspected naval invasion by sea.


Five submarines in particular caused serious damage to Australian and US Navy vessels and merchant ships off the Australian east coast, beginning early May 1942, three of which launched the three mini-submarines that infamously entered Sydney Harbour.


Sydney Harbour invasion


Yet despite the anxiety, in the week leading up to the Sydney Harbour attack, several reconnaissance missions were made by Japanese seaplanes over Port Jackson and the CBD, scoping US and Australian Navy vessels moored around Garden Island, before returning to sea without encountering defensive fire.


Despite inflicting casualties, the three midget submarines that infamously invaded Sydney Harbour on the night of 31 May/1 June 1942 ultimately failed in their mission.

The first, M-27, snagged on a partially-built anti-submarine net; the third, M-21, was detected and attacked in Taylors Bay, Mosman. The two-man crews of both committed suicide in their vessels to avoid capture.


In between, the second, M-24, evaded pursuers, loitered by Fort Denison awhile, then fired its two torpedoes minutes after Garden Island Naval Dockyard lights were turned off for the night, missing its intended targets. Tragically, one of those torpedoes struck a wall on which HMAS Kuttabul, a ferry converted into naval accommodation, was moored, and its explosion caused the troop ship to break apart and sink, killing 21 sailors.


M-24 then vanished, never rendezvousing with the three submarines, I-27, I-22 and I-24, that loitered offshore awaiting its return, and it wasn’t discovered until 64 years later in Nov 2006 by a team of amateur scuba divers. It rests intact on the sea bed at a depth of 55 metres, 5km off Bungan Head, Newport.


M-24 is protected as a war grave, its two crew members still aboard.


The attack on Sydney Harbour was followed by one of the same three Japanese submarines, I-24, firing 10 shells into the Eastern Suburbs shortly after midnight on 8 June 1942 (which coincided with submarine I-21 firing shells into Newcastle in a coordinated attack).


The vessel surfaced just 10km off the coast of Malabar, using the beam from Macquarie Lighthouse to guide its weapons’ trajectory.


The shells destroyed houses in Bellevue Hill and Rose Bay in the Woollahra municipality, but spared civilian lives. Tragically, one American pilot died after he scrambled to fly out to encounter the submarine, crashing his plane on take-off.


As a consequence of the mid-1942 raids on Sydney, Dumble tank stops, dragon’s teeth, iron stakes and concertina barbed wire were installed along Sydney’s beaches. Between Manly and Palm Beach, 300 dragon’s teeth tank traps, weighing approximately 2 tonnes each, were strategically placed along the shoreline, the first group of eight on Fairy Bower Beach (now a rock ledge, the sand long washed away).


Allied military personnel also established encampments in strategic coastal locations, utilising parks and golf courses for training drills, with anti-aircraft guns and powerful searchlights put in place whilst the beaches were closed to swimming.


A 'Dragon's Tooth' anti-tank pyramid among the mangroves at Bayview. Photo: Alec Smart


A row of shoreline defences were also implemented between Port Stephens and Lake Macquarie, to protect the steel-producing port city of Newcastle, vitally important for the manufacturing of military equipment.


In Aug-Sept 2019, after the most severe coastal erosion in 20 years when ocean swells washed away tonnes of sand, a row of dragon’s teeth were exposed again on Stockton Beach.


On 9 Oct 2020, Hunter Water announced: “Over the past two weeks we have removed most of the concrete World War II tank traps from our section of the shoreline at Stockton beach.

“The tank traps were removed to improve the safety of swimmers and surfers using this section of the beach.


185 of an estimated 200 tank traps have been relocated from the water and are being stored on our site. It’s possible the remaining tank traps, which remain buried, will be uncovered in future heavy sea events… We are exploring the potential for reusing the traps in future.”


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