An unusual cloud formation covers the sky above Christchurch this afternoon.

Clouds have formed in parallel lines as a northeasterly wind blows. 

MetService forecaster Daniel Corbett said the formation was a good example of waves or rolls in the mid-level altocumulus or stratus-type cloud.

"The northwest flow across your region of the past couple of days flows over the Alps and the up and down motion downwind of the mountains causes the rolls in the cloud,'' he said.

"In this case, the mid-level layer is fairly saturated and so it forms a wavy cloud sheet. When the layer of air is not as saturated, it can sometimes just form waves or wave clouds."

Meanwhile, tinder-dry Canterbury is set for a welcome reprieve this week as rain sweeps across the country and temperatures tumble.

Firefighters spent last night dampening down more wildfires outside Christchurch after last week's searing temperatures and high winds saw a series of fires destroy homes and kill livestock.

Forecasters say the 30 degree Celsius temperatures will  fall to 16C in Christchurch tomorrow, and rain is predicted for the whole week.

Two scrub fires that devastated Selwyn flared up last night but were extinguished quickly, a Fire Service spokesman said today.

A helicopter with thermal imaging was used to check for hot spots at the two sites.

Cooler temperatures and rain forecast for the region would be a "massive help", but Canterbury was still at risk of fire danger because of the tinderbox conditions, the spokesman said.

There is also a severe-weather warning for the Tararua Ranges, north of Wellington, and heavy rain is expected across the lower North Island and top of the South Island today and tomorrow as a front approaches from the Tasman Sea.

- © Fairfax NZ News

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