The team of ecologists did a desktop evaluation of the region's biodiversity using computerised topographical maps, and have cautioned that nearly all the mapped sites will need to be field-checked.
The exercise excluded all Maori land - at the request of the plan committee - but included some public conservation land to give context to the significance or rarity of native forest types on private land.
Planners reporting to the plan committee say "drive-by" field assessments and corrections of site boundaries should be done first, to eliminate some areas that do not meet the criteria.
The abundance of SNAs raises questions about the best way to manage them, the staff report says.
"To date, policy development was based on the assumption that there would be a large area of 'not significant' vegetation so that SNAs ...would be managed differently in (the new district plan)."
If in fact the vast majority of native bush could be considered 'significant' then that approach might not be appropriate, the planners say.
"However the Regional Policy Statement requirements are very clear: it would be difficult to defend not mapping and identifying SNAs in the plan."
If councils chose not to include SNAs in the plan there was a high risk of court action by conservation interests - as happened when the West Coast Regional Council limited the number of significant wetlands in its Land and Water Plan.
"An appeal by the Department of Conservation and Forest and Bird resulted in the Environment Court requiring a list of wetlands which had not been field assessed - or discussed with affected landowners - to be placed in the regional plan without ... further consultation or notification to the owners."
Government policy and the Resource Management Plan are clear that SNAs must be identified and protected - but there are still options for how that could be achieved through the Te Tai o Poutini Plan, the report advises.
Many councils used non-regulatory measures such as rates relief, education, help with pest control and fencing to support rules about vegetation clearance. But that will be up to individual district councils to decide, the planners say.
What the West Coast plan could offer are incentives for landowners who protect or covenant SNAs, by allowing them to subdivide off additional lots.
"This kind of incentive is included in a number of plans nationally and provides at least a potential benefit to landowners who have an SNA on their property."
The planners have suggested several options for vegetation clearance rules, to be discussed tomorrow by the Te Tai o Poutini Plan Committee of council and iwi representatives.
"These options have been developed reflecting the fact that essentially, most of the native vegetation on the West Coast has been identified as a potential SNA," the report says.
West Coast mayors this month appealed to the Government to rethink its SNA rules.
In a letter to Associate Environment Minister James Shaw the mayors said that with an overall low ratepayer base, the Coast could not keep carrying the weight of the biodiversity policies.
"The QEll covenant system is a proven and successful model at protecting biodiversity on private land, one that landowners and farmers were very happy with.
"The proposed new mandatory process is in our opinion confiscation without compensation and this threat is causing the opposite effect of what you are trying to achieve with landowners clearing land that they would otherwise have looked after and protected."
Greymouth Mayor Tania Gibson said today she was disgusted at recent comments by Shaw, dismissing the concern over SNAs as the grievance of "a few Pakeha farmers down south".
"That was patronising and disgraceful - Mr Shaw is ignoring the fact that half the council leaders in the South Island have written to him about this, and that Ngai Tahu also signed the letter."
The minister had not replied to the West Coast leaders' letter, Gibson said.
