That sentiment has driven other stocks including Sydney-based health supplements firm Blackmores and ASX-listed infant formula maker Bellamy's Australia to record levels.
Milford Asset Management executive director Brian Gaynor, whose firm holds shares in A2, said Australian "momentum investors" were helping to push the stock higher.
"There's an awful lot of momentum investors in Australia - when they see something going they get on to it," he said. "It's very much Australian-driven."
Milford has reduced its A2 stake from 5.4 per cent to 4.8 per cent through the sale of 4.1 million shares, worth $4.3 million, on November 18 and November 23, according to a substantial security holder notice.
A2, which listed on Australia's ASX this year, issued a big upgrade to full-year earnings guidance last week on the back of stronger-than-expected infant formula sales, including in China. The company now expects ebitda of $22 million from total sales of $285 million, up from a previous forecast of $12 million and $267 million, respectively.