Here is some info. If you do a search, some earlier postings will also come up for you here on GW, some of it in the Bulbs forum.
My source is "Grow Nerines" , Kirstenbosch Gardening Series ...
"The exclusively winter-growing species like N. sarniensis require watering from mid-autumn until early summer, followed by a completely dry, summer dormant period."

Also:

"the winter-growing N. sarniensis likes its bulbs to be planted with the necks fully exposed above soil level ..."

Most of South Africa gets its rain in the summer, but the region where N. sarniensis comes from is the Mediterranean climate zone in the southwestern Cape (i.e. winter rain).

If you are a Nerine afficionado, you might consider ordering this little book from the National Botanical Institute in Claremont, South Africa

Hope this helps ...

Only the Cape Province has winter rainfall, most of the rest of South African varies from primarily summer rainfall to split seasons. I have found Nerine bowdenii much easier to grow as a landscape plant in a coastal California setting, where it mostly remains evergreen for me and blooms on cue each October/November in the fall. It also appreciates a well drained soil or it can rot out. I have only done well with N. sarniensis as container plants with summer rest from watering. This may be due to the summer irrigation the whole garden gets, so I have no sunny well-drained spot in my garden which remains completely dry in summer. You might also try the cute little N. masonorum, this does well for me in the ground and pots, and is full of bloom this month.