Market regulators, the NZX and the Financial Markets Authority, say they're confident the approach they took to disclosure of the 1080 contamination threat by listed companies involved was appropriate in the "unusual circumstances", but they will be checking for any suspected market manipulation or insider trading.

Under the NZX Main Board Rule and the Financial Markets Conduct Act listed companies are required to immediately disclose any "material information" concerning their businesses to NZX, unless an exception applies.

The NZX and FMA, in a joint written statement released this afternoon, said they consider that although the alleged threat was treated at all times as genuine by the Ministry for Primary Industries and the police, the nature of the information was maintained in confidence and not sufficiently definite to require earlier disclosure by the potentially affected parties.

Although information regarding the threat was known to specific individuals involved in investigating it, there has been no evidence the market in relation to the relevant issuers was materially influenced by false or misleading information.

And, in fact, the two regulators think the early release of information, which may have been imprecise or not cover all the facts, could have had an unintended adverse impact on trading in their shares.

"Such a release may have created, rather than avoided, the subsistence of a market influenced by potentially misleading information."

Once the NZX learnt that MPI intended holding a media briefing on the threat yesterday afternoon, it halted the securities of the Fonterra Cooperative Group, The A2 Milk Company, and Synlait Milk Limited along with the dairy futures and options traded on NZX's Derivatives market.

The halt allowed for a coordinated release by the parties involved.

As part of NZX's routine surveillance processes relating to price sensitive announcements, trading ahead of yesterday's announcements will be assessed in detail for any suspected market manipulation or insider trading, it said.

There has been mixed trading today in the shares of the three companies that had a trading halt yesterday with the Fonterra Shareholders' Fund unchanged at $5.80, A2 Milk down 3.7 percent to 52 cents per share. Shares in Synlait Milk slipped 0.3 percent to $2.89.