The Australian Federal Police's claim to have seized more than $1 billion worth of ice in push-up bras and craft supplies imported from Hong Kong is under scrutiny, with officers admitting they used an "upper range" figure to make the valuation.

Key points: AFP ice valuations have varied from $666,000 to $2.5m per kilo

AFP ice valuations have varied from $666,000 to $2.5m per kilo Police say seizures will from now on be valued at 'upper range'

Police say seizures will from now on be valued at 'upper range' 720 litres of liquid crystal meth found in bras, craft supplies

720 litres of liquid crystal meth found in bras, craft supplies Four people charged and in custody

Police said four people were charged with importing 720 litres of liquid methamphetamine, in what officers have called the largest seizure of its kind in Australian history.

But the AFP's claim that the drugs were worth more than $1 billion is under scrutiny, with analysis of ice seizures publicised as recently as last month showing police are valuing Monday's haul at nearly four times the price they have used following earlier raids.

The AFP said it came up with the figure using a new "upper range" method of calculating drug prices.

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Police said the 720 litres of methylamphetamine uncovered could have been used to make 504 kilos of ice with a street value of $1.26 billion — working out at $2.5 million per kilo.

However, last month the AFP valued 159 kilograms of ice at $106.5 million, or approximately $666,000 per kilo.

The AFP's NSW Commander Chris Sheehan was asked about the discrepancy by the ABC during a press conference.

"The first point I'd make is that, regardless of the value, once we have seized the drugs, they are essentially valueless because they will never ever, ever, realise a profit for organised crime," Commander Sheehan said.

"In terms of the computation of values, there are a number of different ways they can be calculated.

Ice was allegedly smuggled in bras, bottles of craft glue and painting sets. ( AAP: Dan Himbrechts )

"Typically, what the AFP relies upon is the ACC's Illicit Drug Data Report (IDDR). Now that report comes out annually, and obviously the values change over time.

"The particular value of this shipment has been calculated on the basis of its value sold at a street level, which at that level — a kilogram of crystal methylamphetamine — could be broken into 0.1 gram deals, or 'points', and sold at that value.

"Now, calculating out from there brings us to the $1.26 billion."

Criminologist and former policeman Dr Terry Goldsworthy said he would like to see police be more open about their drug valuation methodology.

"It needs to be consistent. It's always good for a headline to say 'we've got over a billion dollars worth of drugs' but there needs to be some realistic methodology behind that and they should probably make that methodology known," Dr Goldsworthy said.

"It's no secret, the ACC puts out data on prices of drugs and street prices. I'd like to see the AFP, or whoever makes that determination, put forward how they actually calculated that."

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In a statement released later on Monday, the AFP admitted that its calculation method had recently changed.

"A decision was made to standardise AFP drug calculations from the most current IDDR (using the upper range figure) as previously it was mid-point and state by state based," the statement said.

"Now we use a single point of reference."

Valuations aside, police have another number they are focused on — the four arrests.

Three men and a woman from Hong Kong and China have been charged with major commercial drug supply offences.

Law enforcement officials say the liquid methylamphetamine was artfully concealed in paint-by numbers kits, glue bottles and push-up bras.

The four people charged over the seizure remain in custody, and face life in prison if convicted.