Synlait fell 7.4 per cent to $7.27 and A2, its infant formula partner, declined 4.5 per cent to $7.62.
The shares have soared 135 per cent and 259 per cent respectively this year.
"There's been a degree of profit taking from investors for a number of days now," Williamson said.
"As the prices slip it could fuel further selling. Both have been overdue for a correction."
It was too early to say whether the decline signalled a change of sentiment toward the companies, he said.
F&P Healthcare is a key beneficiary of a weaker dollar because it makes most of its sales overseas.
The company's share price gained 2.8 per cent to $13.44 and is edging back toward the record $13.49 it reached late last month.
Fletcher Building fell 1.7 per cent to $7.07 and has fallen 32 per cent this year, a period in which it has changed its chief executive and apologised to shareholders after being unable to control cost blow-outs at two major construction projects.
Trustpower fell 1.7 per cent to $5.87, and Ebos Group fell 1.4 per cent to $17.35.
Among gainers, Spark New Zealand rose 2.8 per cent to $3.71, Xero rose 1.9 per cent to $34.70 and Air New Zealand rose 1.5 per cent to $3.37.
Tower fell 2.5 per cent to 78.5c. Suncorp Group-owned Vero Insurance has ditched a planned appeal of a rejected takeover bid for Tower after the New Zealand insurer terminated their arrangement.
Aquaculture producer New Zealand King Salmon Investments rose 3.1 per cent to $2.36.
Evolve Education was unchanged at 78c after the company said it was on track to post a first-half profit of $7m not including a non-recurring expense item of $3m.