Stamp 4 - Maternity leave more than 6 months overseas

Hi,
My wife is on stamp 4 visa and it expires in September. She is on maternity leave and joining work in August. She has gone to India in December 2020 and planning to return by july. Will there be any problem during renewal in September as she wud have stayed more than 90 days by that time?
Or any problem she would face in immigration when returning in July.
My child is irish citizen and I am due to get my citizenship certificate soon.

And also would there be any tax implications for staying more than 183 days? However she is on unpaid leaves for 4 months.