The stock exchange temporarily suspended trading in 46 shares yesterday and then briefly closed the market in the latest in a string of technical glitches.
It was the market's third time down in four months.
Yesterday's problem started with a mistake in the weekly diary published by NZX.
The diary saidcalculations for a one-for-eight bonus issue by Scott Technology would see fractions rounded up.
In fact, Scott wanted the fractions rounded to the nearest number, either up or down.
Share registry BK Registries, of Ashburton, relied on the diary - rather than Scott's notice to the stock exchange - and the resulting miscalculation added 1300 shares to the 2,754,442 of the bonus issue.
To "back out" the mistake, and add the correct data, BK disconnected from the FASTER trading system.
That led to the announcement at 2.31pm that NZX had suspended trading in 46 NZSX and NZAX shares handled by BK, including Briscoe Group, Infratil, Kiwi Income Property Trust, Telstra and Tourism Holdings.
Then, after BK corrected the Scott data, the whole market - NZSX, NZDX and NZAX - had to briefly close.
NZX chief information officer Chris Corke said there was a daily upper limit for "status changes" - such as "opened" or "suspended" - to securities and the limit needed to be lifted because of the to-ings and fro-ings over the BK shares.
The unanswered question yesterday was why Scott's stock was not put in a trading halt for the day, leaving BK to rework the figures overnight without disrupting the market.
Corke said disconnecting from FASTER was BK's choice. "It wasn't our choice".
However, at BK, Peter Wilson pointed to the exchange's so-called "zero tolerance" policy.
Corke said the exchange was "considering a number of options" to prevent future failures and expected to make an announcement on the topic before year's end.
At brokerage Hamilton Hinden Greene, partner Grant Williamson said: "It's always of concern when investors and brokers can't trade on the market."
In one of the previous incidents, a technical hitch stopped trading just as broker ABN Amro Craigs was making a surprise stand in the market for Wrightson shares.
