At 6pm, MetService figures showed Wanaka as the highest in the country at 29.6degC, while Dunedin was the coldest at 13.6degC.
Mr Mercer said low cloud meant Dunedin, which reached a high of 19degC today, did not get the same heat as its inland neighbours.
Central Otago could expect similar temperatures tomorrow, although possibly a few degrees lower, with a chance of isolated rain in the morning.
Alexandra and Wanaka were set to hit 29degC, while Queenstown was expected to reach 26degC.
The low cloud was expected to linger in Dunedin, but it could potentially clear on Saturday.
Oamaru had a high of 18degC today, while Balclutha reached 19degC.
Tomorrow, Dunedin and Oamaru could expect a high of just 17degC.
Niwa said it was expecting summer to be hot and dry for much of the country.