THE 50-minute long lockdown of Singapore's airspace to commercial planes on Tuesday night was caused by a small Australian-registered plane, The Straits Times reported.
The plane eventually landed at Changi Airport, under escort from two Republic of Singapore Air Force (RSAF) fighter planes.
Two Australians, who were the only people aboard the Cessna 208 Caravan float plane - which can land on water - are now assisting a police probe into the security alert.
The alert happened during the busiest time for Changi Airport's runways.
Aviation officials estimated yesterday that the closure of Changi Airport for 50 minutes could have caused airlines to
rack up fuel costs amounting to thousands of dollars.
In total, 16 planes circled Singapore awaiting permission to land.
The incident also caused a Royal Brunei flight, inbound from Bandar Seri Begawan in Brunei, to make an unscheduled stop at Senai airport in Johor.
This was because it was low on fuel.
Brunei's flight BI 423 landed in Singapore later, one hour 45 minutes late.
Six departing flights were also delayed.
The delays were triggered by the closure of Singapore's airspace between 7.10pm and 8pm as the RSAF launched two fighter planes to intercept a mystery plane.
Tuesday's airspace lock down is the first reported scramble since 2003.
The last incident took place in August that year when RSAF Super Skyhawk warplanes intercepted a civilian plane that attempted to land at Tengah Air Base.
The Straits Times understands that the Australian-registered plane in Tuesday's incident began its flight from Koh
It's an island off the East of Thailand, which is famed for its beautiful beaches.
A check showed that it was bought this month by Ms Mary Cummins.
She co-owns a tourist adventure flight company with Mr Rhys Thomas.
He is a pilot who once flew with Australian airline, Ansett.
Under previous owners, the float plane, which can carry 12 people, was flown by an airline in Koh Samui.
Ms Cummins and Mr Rhys could not be reached yesterday for comment.
The Caravan is usually based at Broome Airport, in Western Australia.
It ferries tourists on joyrides to scenic spots near the remote outback town.
As sundown approached on Tuesday, radars operated by the RSAF's Air Defence and Operations Command tracked up the Caravan as it flew towards Singapore.
The time was fast approaching the busiest spell in Changi Airport's work day.
This peak period in aircraft arrival and departures usually sees 30 to 40 aircraft movements an hour from 7pm to midnight.
Scanning the crowded air lanes which lead to Singapore, RSAF air defence personnel quickly established that the Caravan did not have an approved flight plan.
After the Sept 11, 2001 terrorist attacks in the United States, security forces worldwide have been especially wary of unidentified aircraft flying towards populated areas.
These could be piloted by terrorists intent on crashing the plane to cause mayhem.
At 6.42pm, a pair of RSAF twin-seat F16D warplanes thundered off an air base to intercept the Caravan.
The gun and missile-armed warplanes signalled to the Caravan to land at Changi Airport's central runway just before 8pm.
Airport police immediately surrounded the plane and spoke to the two Australians.
The Singapore Police said yesterday that "two foreigners" aboard the plane are assisting with investigations.
The police could not reveal more as its probe is underway.