"Probably most of the country will get heavy rain warnings, or at least a watch for Thursday and Friday.''
It was a good time for people to check their gutters and drains.
"Because it's so long since there has been significant rain, there could well be a lot of debris (in them).''
At present, it was expected to hit New Zealand on the West Coast, near Hokitika, on Thursday, but its track was uncertain.
"The strongest winds and heavy rain will be on the west of the South Island,'' Mr Mercer said.
For the east, the worst of the storm is expected to be experienced from Kaikoura to Clutha.
The strong northerlies to northwesterlies the storm brought would be followed by a strong, relatively cold southerly change also expected to bring heavy rain.
The southerly change was expected to "move off quite quickly'' and should have mostly cleared by the end of Friday.
Southern farmers should receive relief from the heatwave, but the question is whether the expected deluge will be helpful or destructive.
Southern meat works are working at full capacity as drought-hit farmers destock; some winter feed crops have already failed and irrigation is being scaled back because of falling river levels.
Federated Farmers Otago president Phill Hunt, of Wanaka, believed the coming weather could provide some much needed "intense rainfall'' on parched paddocks.
However, he was concerned the ground was too dry to absorb moisture and it could run off.
"There's not the vegetation on the ground to hold the moisture; it could potentially cause surface flooding,'' he said.
Although many lower South Island farmers are destocking, upper South Island and North Island farmers have had the luxury of some recent rain, allowing them to fatten stock over the coming weeks.
Mr Hunt said stresses for farmers were beginning to mount. He urged all farmers who might see family or neighbours becoming stressed to seek assistance from the Rural Support Trust.
