Market standard is 30% extra compare to similar perm position, pro-rata basis.
Can claim expense for pretty much all the things, if there’s any.
3% agency fee (bit cheaper if you manage your tax bits and indemnity insurance by yourself)
8.3% annual leave (20 working days = 4 weeks)
4.16% personal leave (10 working days = 2 weeks)
in total: 24.96%
IMO, 5% income boost + some additional tax deduction doesn’t really make much sense.
Not sure it’s worth leave perm job.
dn;tl: It’s not simple 30% salary bump trade off for job security