"We didn't know what to expect," he recalled.
"But I thought before I went I'd have a bath and a full meal because I didn't know when I'd get the next one."
The teenager became a soldier in the Schoolboy Company, part of the much bigger First Auckland Regiment occupying the racecourse.
His new home was a tent, his bed a palliasse, a straw-stuffed mattress: "You didn't expect inner springs."
At night Watt manned a machine-gun post, watching the skies in case Tokyo ordered an airborne invasion.
"I think the army had no idea what was going to happen.
"Everyone got a huge shock when the Japanese attacked the Americans. If they'd come after us we would not have withstood the onslaught."
Japan followed its lightning blow against the Pacific Fleet with an attack on the British colony of Malaya. Two Royal Navy battleships were destroyed.
New Zealand pilot Geoff Fisken described "a grey metal bow sticking out of the sea, surrounded by an oil slick and many bodies".
It was clear Japan's aspirations were much wider than China alone, where it had waged war since 1937.
New Zealanders began to ask: could the Japanese head down here?
The country looked vulnerable. Pearl Harbour was closer to the US than Japan. The Japanese Navy was free to strike at long distance - the crippled Pacific Fleet no longer stood in its way.
New Zealand's focus had been in helping Britain's battle with Germany. The Second New Zealand Expeditionary force - about 30,000 men - was tied down in North Africa.
At home, guns, bullets and tanks were scarce. The Schoolboy Company were told a World War I Lewis gun was the best they could expect.
"Obviously the army had searched their stocks and come up with the best they could find," recalled Cedric Watt.
Anxiety deepened in March when a small seaplane launched from a Japanese submarine flew over Wellington and Auckland.
Hidden anti-aircraft guns, including one on the Ponsonby Post Office, remained silent.
The submarine headed for Fiji. A second entered the Hauraki Gulf, again found nothing, and headed for Sydney where it launched mini-subs that attacked two ships.
The massacre at Nanking - where Japanese troops used Chinese people as bayonet practice and raped children and women as old as 70 - was widely known.
"We were prepared to act quickly and with courage - die rather than surrender," said a Hawkes Bay woman.
Major T.H. Melrose, a home guard commander, began a movement called "Awake New Zealand". He wanted every adult to be trained and armed. Some made their own
"jam tin" hand grenades and trench mortars.
The Government asked the army for its assessment. Its stark conclusion: just one Japanese division of 20,000 men could take New Zealand.
Had the generals known that Japan was printing invasion money and maps in Japanese, concern might have been even higher.
Wartime Prime Minister Peter Fraser's cabinet swung into action. More than 20,000 territorials were mobilised and troops were sent to Fiji, where the army expected the first Japanese landing would occur.
A tank was made from a bulldozer. Called the "Semple Tank," it was named after Arms Minister Bob Semple.
The army rejected it as too slow and heavy. But a New Zealand-designed machine gun, the Charlton - made from recycled World War I rifles - proved workable. Hundreds were made. Dummy weapons were made from wood.
Cedric Watt remembered moving from Avondale to a farm at Castor Bay on the North Shore. A barrier made from a log and designed to fall across the main road was built to halt an invasion from the north.
The US made plans to base B17 "flying fortress" bombers at Whenuapai and at a new airfield near the Rakaia River mouth in Canterbury.
One aircraft crashed at Whenuapai in early 1942. The South Island runways were completed but not used.
Until they took delivery of American Kittyhawk fighter aircraft, the air force resorted to arming Tiger Moth trainers.
Coast-watching stations were set up around the country. The navy requisitioned private launches and World War I veterans patrolled the coastline.
Cities started to look like they were at war.
Windows were blacked out, streetlights dimmed and shielded, if they faced the sea and buildings sandbagged.
Barbed wire protected beaches. Air raid trenches were dug in schoolgrounds and backyards.
Auckland City Council dug nearly two miles of trenches, expanded natural caves in Mt Eden and opened up a disused tunnel near the Domain.
Nine months after Pearl Harbour, 58,000 Aucklanders could be sheltered underground, 20,000 of them beneath Albert Park.
The Government appointed James Fletcher to oversee construction of massive coastal defences.
Concrete gun emplacements were built at Stony Batter on Waiheke and Whangaparaoa's Army Bay.
If there was one single event that kicked invasion plans off the table it was the Battle of Midway in June 1942, which thanks to the cracking of Japanese codes, good tactics and some luck, went America's way.
With four aircraft carriers sunk and hundreds of planes destroyed Japanese naval power was demolished.
Only a month previously, the Japanese navy had been weakened at the Battle of the Coral Sea.
But what if Midway had gone the other way, when New Zealand clearly was vulnerable?
The United States naval power would have been checked, although they would have been able to move aircraft carriers from the Atlantic, just as they moved battleships after Pearl Harbour and repaired damaged ships.
Practicality probably tempered Japanese dreams.
Though its navy was keen to invade Australia, the Japanese army was against it, because occupation would have meant moving 10 divisions, many from their Russian front.
Even so the Japanese bombed Darwin more than 60 times.
It would have helped to bring the New Zealand Division home, although Britain was against it.
When Australia brought some troops home from the Middle East, the news was censored.
Instead New Zealand had a friendly invasion, when two US divisions arrived in June 1942 on their way to fight the battle of Guadalcanal.
With the country a base for American troops, the threat of Japanese attack receded.
Still, a nervous Government banned the media from mentioning the welcome US presence - even the fact the smart young American soldiers were buying all the flowers - until late in the war.
- Additional reporting: Andrew Stone
