The Tasman fire that started yesterday afternoon in Pigeon Valley has doubled in size overnight and spread to cover 1870ha within a perimeter of 20km by 3am.

The fire raging near Nelson has destroyed homes and forced dozens of families to evacuate, prompting a Civil Defence emergency to be declared in the Tasman Region by mayor Richard Kempthorne.

Here's what you need to know about the blaze:

• The Tasman fire that started yesterday afternoon in Pigeon Valley has doubled in size overnight and spread to cover 1870ha within a perimeter of 20km by 3am.

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• Fire crews are hoping a change from a southerly to a northerly will help slow the fire spread.

• More than 150 homes have been evacuated or self-evacuated with people registering with the Civil Defence Centre in Wakefield.

• Overnight 12 crews fought the blaze on an increasing front protecting as many structures as possible and minimising the burn on grasslands.

• There have been no reports of injury or death.

• People have been evacuated from Eves Valley, Golden Hills Rd, Redwood Valley Rd, Mallings Rd and Greenacres Rd.

• Firefighters today plan to mount an aerial assault on the fire helicopters and fixed-wing aircrafts and on the ground heavy machinery would be used to create firebreaks.

• The cause of the fire is not yet known.

Fanned by southerly winds over most of the night the fire has caused material damage from Pigeon Valley north to the Inland Moutere Highway and Waimea West Rd.

If people feel unsafe or unsure in any way, they are advised to self-evacuate to the Civil Defence Centre at St John's Church on Edward St in Wakefield.

Nelson Tasman Defence Group controller Roger Ball said anybody evacuating should take essentials like food, water, clothing, medicine and pets.

Those self-evacuating were advised by Fire and Emergency New Zealand to secure their homes to prevent ember entry.

That meant taking all flammable items like outdoor furniture inside.

"The threat to your house will be embers. Think about where an ember might light."

Blocking access underneath the house would also decrease the odds of an ember sparking a fire in a house.

PHOTOS OF THE BLAZE

Mireille Verhoef, from the Netherlands, caught this snap from her campsite in Nelson. Photo / Mireille Verhoef
Mireille Verhoef, from the Netherlands, caught this snap from her campsite in Nelson. Photo / Mireille Verhoef
The glow from the fire taken at 8.52pm around 10km away in Mapua. Photo / Denis La Touche
The glow from the fire taken at 8.52pm around 10km away in Mapua. Photo / Denis La Touche
The sun hidden behind a smokescreen made for an interesting sunset. Photo / Jack Burden
The sun hidden behind a smokescreen made for an interesting sunset. Photo / Jack Burden
The view of the fire from the tarmac at Nelson Airport. Photo / Cate Creemers
The view of the fire from the tarmac at Nelson Airport. Photo / Cate Creemers
Strong winds and dry conditions helped the fire spread. Photo / G and T Orr
Strong winds and dry conditions helped the fire spread. Photo / G and T Orr
Police are urging those nearby to evacuate. Photo / Stephen Cox
Police are urging those nearby to evacuate. Photo / Stephen Cox
The glow from the fire was easy to see for Denis La Touche around 10km away in Mapua. Photo / Denis La Touche
The glow from the fire was easy to see for Denis La Touche around 10km away in Mapua. Photo / Denis La Touche
Rilton Medeiros' view of the fire at 9.11pm. Photo / Rilton Medeiros
Rilton Medeiros' view of the fire at 9.11pm. Photo / Rilton Medeiros
Mireille Verhoef, from the Netherlands, caught this snap from her campsite in Nelson. Photo / Mireille Verhoef
Mireille Verhoef, from the Netherlands, caught this snap from her campsite in Nelson. Photo / Mireille Verhoef
A view of the smoke pumping out from the fire at Makee Reserve. Photo / Ransom Adventure
A view of the smoke pumping out from the fire at Makee Reserve. Photo / Ransom Adventure
Taken from a vineyard in Brightwater. Photo / Mik Reeves
Taken from a vineyard in Brightwater. Photo / Mik Reeves
Smoke billowing over Nelson at sunset. Photo / Rebecca Bowater
Smoke billowing over Nelson at sunset. Photo / Rebecca Bowater
Large volumes of smoke pumped over Takaka Hills. Photo / Amanda Hunter
Large volumes of smoke pumped over Takaka Hills. Photo / Amanda Hunter
Smoke shielded the sun from UB40 spectators at Trafalgar Park. Photo / Jan Hania
Smoke shielded the sun from UB40 spectators at Trafalgar Park. Photo / Jan Hania
The glow from the fire was easy to see for Denis La Touche around 10km away in Mapua. Photo / Denis La Touche
The glow from the fire was easy to see for Denis La Touche around 10km away in Mapua. Photo / Denis La Touche