It's being predicted primary teachers will vote to strike for a full day.

Teachers and principals of the Educational Institute are voting on whether to extend the August 15 strike from three hours to a full day.

NZEI president Lynda Stuart says she's getting a clear vibe from union members.

"As we went through the paid Union meetings up and down the country, the mood was certainly an overwhelming rejection of the Ministry's offer, and a move towards a full day strike."

Voting on the teachers strike closes tomorrow night, with results expected the following day.