While bushfires are an anticipated event with each fire season, it is important that the extent of likely annual fire weather conditions and those of more extreme conditions are identified as part of the preparation of communities for those fire events. In preparing defendable space arrangements (often referred to as asset protection zones), for both existing homes and when planning for future urban expansion, the characteristics of fire weather conditions are critical. Historically, extreme value analysis has been used for floods, storms and wind, however, such analysis has not been routinely undertaken for fire weather. This may be because, historically, fire weather is a composite of differing parameters which, at their extreme, individually may not be related to bushfire. Fire danger rating and index systems generate non-dimensional parameters and in Australia, this has generally focussed on forest-fire danger indices or grassland-fire danger indices.

While it is difficult to ascertain individual bushfire extremes related to traditional parameters like wind speed, temperature and relative humidity, the use of the extreme value distributions are a suitable process for fire danger indices in Australia. This can then be applied to deterministic fire behaviour assessments, whether through the identification of rate of spread, flame height (or length), intensity or suppression effort.

This paper offers new insights in the use of three extreme-value assessment techniques for both MacArthur and Project Vesta equations which, when combined with fuel assessments, can be used to determine comparable defendable space for buildings under both existing and planned urban developments in NSW. Treatment options can be more effectively determined and quantified under such an approach so as to better balance resident safety as well as recognise environmental assets. For planning purposes, improved-design bushfire scenarios can be applied.