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Financial Risk Management - Sensitivity Analysis for Types of Market Risk (Details) - NZD ($)
$ in Thousands
Jan. 31, 2020
Jan. 31, 2019
Jan. 31, 2018
10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic $ 179 $ 200  
10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (179) (200)  
US [Member] | 10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (594) (954) $ (1,509)
US [Member] | 10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic 594 954 1,509
AUD [Member] | 10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (1) (5) (805)
AUD [Member] | 10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic 1 5 805
GBP [Member] | 10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (16) (1) (175)
GBP [Member] | 10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic 16 1 175
EUR [Member] | 10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (42) (32) (136)
EUR [Member] | 10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic 42 32 136
HKD [Member] | 10% Increase [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic (1) (14)
HKD [Member] | 10% Decrease [Member]      
Statement Line Items [Line Items]      
Risk exposure associated with instruments sharing characteristic $ 1 $ 14