Income taxes - Reconciliation of Income Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Jan. 31, 2020
Income Tax Disclosure [Abstract]
Income before income tax expense	$ 20,816	$ 24,625	$ 16,488
Tax at South African statutory rate of 28%	5,828	6,895	4,617
Tax effect of:
– Income not subject to tax	0	(41)	(42)
– Non-deductible expenses	235	423	497
– Non-deductible/(non-taxable) foreign exchange movements	4,085	3,441	(2,169)
– Investment in subsidiaries	42	(37)	(31)
– Withholding tax	0	56	56
– Utilization of previously unrecognized tax losses	(195)	(385)	(483)
– Foreign tax paid	623	272	217
– Foreign tax rate differential	(213)	(290)	(196)
– Recognition of previously unrecognized tax losses	(11)	(262)	(86)
– Tax losses not recognized	75	18	40
– Under-provision prior years	138	157	27
– Tax incentives in addition to incurred cost	(897)	(448)	(258)
– Stock based compensation	0	(113)	(81)
– Transfer pricing imputation	65	6	260
– Imputation of controlled foreign company income	119	177	182
– Other	(65)	(54)	(14)
Income tax expense	9,829	9,815	$ 2,536
Deferred tax asset	$ 3,108	$ 3,197		$ 500