Spinach growing in sandy soil will need daily irrigation at most times of year and twice or more daily in hot weather. It will not tolerate waterlogging.
Comprehensive information on the principles of irrigating vegetable crops in WA can be found on the vegetablesWA website. Irrigation rates for your specific location and circumstances can be calculated by using the irrigation calculator.
Spinach has a higher salt tolerance than most other vegetables and can be grown successfully with water of an electrical conductivity (salinity) level as high as 270mS/M. It can tolerate water up to 635mS/M.