The former Otago Boys' High School principal retired at the end of 2014, but has been acting principal of a primary school and a secondary school in Kaikoura for the past year, after placing his name on the Ministry of Education's emergency principals list.
Mr Rennie said WBHS commissioner Nicola Hornsey was looking for an ''experienced principal'' to take temporary leadership of the school until a permanent rector could be appointed.
The school is advertising nationally to fill the post.
Applications close on Monday.
Mr Rennie said Ms Hornsey ''was given my name, and she approached me''.
''Certainly, I'm happy to be there for term one next year,'' Mr Rennie said.
''I'm basically retired, but I have skills I've built up over the years, and I'm really happy to be asked to be in a school of the stature of Waitaki Boys'.
''Things haven't been easy for them lately, but it's still a great old school. It's nice to be able to help.
''Hopefully, I can make a contribution in bringing the community back together because it's a school that in the past has had great support, and hopefully in the future we'll re-establish that.''
Mr Rennie will start in the role at the end of January.
He said he had been really excited about his work with the Kaikoura schools and was equally excited about the work ahead at Waitaki Boys'.
It had given him ''a new lease on life''.
Ms Hornsey said she had already fielded several inquiries and sent out information packs.
Interviews would not take place until the new year and she hoped the new rector would start by term 2 next year.
After rector Paul Jackson's announcement he would leave at the end of the year, the school sought approval from the Ministry of Education to advertise the vacancy for a new rector with a ''principal recruitment allowance'', meaning the school could offer the successful candidate an additional $50,000 per year.
Schools with ''significant challenges'' could submit an expression of interest to the ministry to request the allowance for recruitment.
However, Ms Hornsey said, the school had been told by the ministry that before it could be deemed eligible for the allowance, it had to ''test the waters'' and advertise the job.
