November's rainfall was only 21 per cent of normal, meaning there were dry conditions leading up to the first weekend of December, when temperatures reached up to five degrees higher than usual.
Although there is "continuous monitoring" of Napier's reservoirs through the electronic SCADA (Supervisory Control and Data Acquisition), the usual pump automation system did not occur.
Council director infrastructure Jon Kingsford said the reservoirs had alarms to trigger at different levels - whether the water level was too high, low, or ultra low.
"They're monitored 24/7 and there is always someone on call," he said. "If it's low, the system then automates the pumps to kick in without any manual intervention.
"It works well under normal circumstance, however, the Sunday night events were extreme. We are going to look closely at this incident and act on any learnings from it."
The SCADA system had shown relatively normal water levels on Friday and Saturday. The "great drawdown" of water occurred between Sunday morning and Monday morning.
After this discovery on Monday, Mr Kingsford said staff decided restrictions needed to be implemented, so the council's "three waters team" began working with the communications team on a public message.
"During the course of working that up we realised it was actually more significant an event than what we first thought, and so we thought the message then had to be adapted.
"We released it as soon as we were able …it just takes time to get that messaging right."
The council's communications and marketing manager Fiona Fraser said their channels were "highly effective", and they had moved through their communications programme quickly on Monday.
The Facebook post which broke the news of the shortage reached more than 50,000 in a few hours, "which is reflected in the immediate response, with people limiting their usage pretty much straight away".
Ms Fraser said the council's website was changed to reflect the restrictions being in place before this, and a media release went out later on the restriction and conservation campaign.
"We conducted half a dozen national and local media interviews across the afternoon, and another six [on Tuesday] with items on all websites within an hour or so of the restriction being enforced.
"By 6pm it was pretty hard to have missed the news – it made headlines across NZ."
The council were faced with another obstacle on Monday - with some residents finding black water was coming out of their taps after new bores were brought online for the first time in the network.
Mr Kingsford said staff had been working overnight to remove the discoloured water, which occurred as the main water column through the network changed direction.
"So that can occur due to chlorination and or changes in water direction in the pipe network. While that is obviously undesirable it can happen from time to time and unfortunately it occurred this time."
