These are SG 372 (3d) and 373 (6d) - from a 4 stamp set issued 1 to 17 November 1906 to commemorate the New Zealand Exhibition in Christchurch. The other two stamps in the set were SG 370 (1/2d) in single colour green and 371 (1d) in single colour vermilion. The used set of 4 had a catalogue value in 2016 of GBP 375.
As for the exhibition itself, there is plenty of info on Google.
Some copies of the 1d had been printed in claret prior to issue. A sheet of these was unintentionally added to stock of stamps for sale at the exhibition. Hence the claret - SG 371a - has a CV of GBP 8,000 mint/11,000 used.