There was generally a lag between the GDT price of a product and the retail price, Penny said.
"It looks like demand is very strong and suppliers are really struggling now," he said.
"What we have seen here is a sea change in demand for milk fat."
At this morning's auction, butter hit US$6004 a tonne - up 3.4 per cent since the previous auction early this month and more than double the price paid this time last year.
Fonterra, the world's biggest dairy exporter, now had the choice as to whether to produce more butter than its main product, whole milk powder.
"Fonterra's recent added production flexibility means it can more easily optimise its production mix," Penny said.
Whole milk powder - which is the key product for determining Fonterra's farmgate milk price - gained 0.3 per cent in value to US$3114 a tonne.
Prices overall continue to be in a holding pattern.
AgriHQ chief analyst Susan Kilsby said strong consumer demand for natural, full-fat products such as butter was here to stay.
"This strong demand along with limited supply is keeping buyers of dairy commodities paying current high prices - though they aren't necessarily happy about paying so much for it," she said.
Cheddar gained 1.6 per cent at the auction, lifting to an average price of US$4112/tonne.
Kilsby said cheddar prices were expected to come under pressure as more product became available from both Europe and the US.
Skim milk powder prices fell another 3.2 per cent to US$2024 a tonne, reflecting stock piles in Europe that were built up from earlier price support schemes.
"The volume of skim milk powder storage, including that in intervention in Europe, continues to put pressure on this part of the market," Kilsby said.
Some private forecasters expect ongoing firm prices to translate into a $6.75/kg milk price as the season progresses.