Huge areas of Tauranga's once bustling civic centre has emptied out into a gloomy unsafe expanse of offices awaiting leak proofing and seismic strengthening.

The discovery of toxic mould after a staff member fell sick three months ago triggered a massive council response, the first phase of which was completed over the weekend.

It signalled an important milestone in Project Clean in which more than 580 shifts involving staff had taken place from areas that tested positive to mould. Large numbers of staff needed to be shifted twice, once to temporary safe areas such as Baycourt and then into long-term leased accommodation in the downtown.

Project manager Terry Wynyard said 400 staff had been shifted, with the last shift taking place place over the weekend into an area created by taking space off library administration - removing the final obstacle to water tightness repairs that could cost up to $2 million.

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A water tightness report from Prendos and a seismic strengthening report from Beca consultants were expected in the next week to 10 days, he said. The Beca report would then be peer reviewed before both reports, with recommendations, went to the council for decisions.

The Prendos investigation included an exterior examination of the civic building last month in which dye-coloured water was pumped into anything that looked suspicious in order to see where the water emerged inside the building.

Mr Wynyard said the problem arose when litres were poured in and nothing appeared on the other side. "There was a bit of that, but it was not unexpected."

The council planned to use this year's $1 million rates surplus to help fund unexpected costs estimated to reach $1.37 million by June 30.