Australian shares reversed early gains to end flat on Tuesday and the NZX sagged after police said threats had been made to poison baby formula from New Zealand milk producers.

New Zealand’s benchmark NZX50 index closed 0.2 per cent lower at 5,887.7. Fonterra shed 1.2 per cent, while Synlait Milk fell 1.7 per cent to its lowest in over 18 months.

Environmental activists were suspected to have made the threats in an attempt to halt the use of an agricultural poison on pests such as rats and possums.

Australia's benchmark S&P/ASX 200 index closed a touch firmer at 5,824.2 as early gains mostly evaporated.