"We believe measures the Government has taken since 2009 have resulted in the supply of precursors being constrained and it is reflected in the statistics," he said.
But a working group set up by Prime Minister John Key was warned that although smaller amounts of pseudoephedrine were being stopped, this did not necessarily mean less was being smuggled in.
"The ongoing decline in the quantity of precursors seized is likely to be a reflection of a change in modus operandi by the syndicates involved, rather than an indication of reduced quantities entering New Zealand," according to October's Tackling Methamphetamine report from the Department of the Prime Minister.
Hundreds of kilograms of pseudoephedrine seized in Operation Ghost were disguised as bags of breadcrumbs or starch shipped from China and destined for a yum cha restaurant in downtown Auckland.
A spokesman for Mr Williamson said he stood by his statement.
"There's clear evidence that the methamphetamine market is now smaller than it once was and the efforts of the Government are having an impact."
There is evidence the number of methamphetamine users has halved from 2.2 per cent of the population to 0.9 per cent in the five years to 2012, says a Ministry of Health survey.
But a classified police report obtained by the Weekend Herald estimates that 75 per cent of all methamphetamine is now consumed by 11 per cent of "heavy" users.