Have you found the 255 character limit for your SPF record to be too short? The way around this limit is to merge multiple records together using the "include" feature of the SPF ruleset:
For example: an existing record needs to be modified for a list of IPs.
example.com.au. TXT "v=spf1 +a +mx +ip4:188.8.131.52 -all"
Create a new txt record:
_spf1 TXT "v=spf1 +ip4:184.108.40.206 +ip4:220.127.116.11 include: m.google.com ~all"
Once the new record is created modify the original record and use:
example.com.au. TXT "v=spf1 +a +mx +ip4:18.104.22.168 include: _spf1.example.com.au -all"
Once this DNS change has been made and propagated your SPF records will now include both rulesets.