It's the gargantuan ghost ship contract that's been hiding in plain sight for months.

Key points: Defence appeared to be paying a whopping $307m to UK consultants for "advisory services"

Defence appeared to be paying a whopping $307m to UK consultants for "advisory services" The price tag was almost enough to build a brand new warship for the Navy

The price tag was almost enough to build a brand new warship for the Navy It turns out a misplaced decimal point and several stray zeros were to blame

A whopping $307 million paid by the Defence Department to a company in the United Kingdom, under a procurement method now infamous for its lack of transparency: a "limited tender".

Defence types, contractors and government waste-watch fanatics have been wondering for weeks — what on Earth did $307,579,624 buy?

In late December, the contract for "Shipbuilding Benchmarking and Advisory Services" was quietly published on the Commonwealth's public database, AusTender.

The recipient of the mysterious pre-Christmas bonanza was British-based engineering consultancy firm Haskoning DHV UK Limited.

Was it a sweetheart deal done with a long-time supplier, or a massive conspiracy to outsource Australian shipbuilding jobs?

Whatever it was, the $307 million price tag was almost enough to build a brand new warship for the Navy.

The Department of Defence AusTender website advertising a $307 million contract. ( AusTender )

Earlier this month, the ABC put some formal questions to Defence about the eye-watering figure.

Almost a fortnight later the department has finally offered an explanation.

It turns out a misplaced decimal point and several stray zeros were to blame.

"Through an inputting error, a contract amendment was issued with a figure of $307,579,624," the Defence Department said in a statement.

"This was immediately corrected back to the original amount of $291,798.79."

Just last month, Defence revealed to Parliament that the department had drawn up a total of 23,000 contracts in the past financial year, totalling $28 billion.

Why does a stray decimal point matter?

Over the next decade, the Defence Department is due receive an extra $200 billion to fund the biggest build-up of military equipment the nation has ever purchased.

Get the decimals wrong and that either becomes an eye-watering $2 trillion, or a vanishing $20 billion — one for the Department's many bean counters and auditors to keep an eye on.