The novice drivers will race in the 0-1300cc category and Johnston (32) said the pair did not have any firm goals other than keeping their Toyota Starlets on the road and out of the ditch.
"It would be a good idea not to prang the car because it is leased, although my husband does own a panel beating business," Johnston joked.
"Our ultimate goal is to finish the rally with no time penalties and a car which has no damage.
"We'd both like to say we beat each other, but ultimately we'd like to beat some boys and show that the girls can actually do it."
The Dunedin sisters were raised in a motorsport-mad family and are no strangers to rallying.
Johnston has done some co-driving for her husband and Harper (34) entered a rally as a driver 16 years ago.
But the sisters put their rallying careers on hold while they started families.
"My sister and I were brought up with motorsport," Johnston said.
"My father and mother have been into motor racing since they were teenagers and ever since we've been old enough to run down a gravel road we've been out with them."
The passion remained and plans to make a return were hatched during a family barbecue.
"We were having a barbecue one night and decided that we would challenge each other. One of the rules were that our co-drivers had to have never been racing before. So we've got two novice co-drivers."
Johnston talked her friend, Jody Jenkins, into it and Harper has enlisted Paula Dore's help.
While it is not the first time sisters have competed against each other in a rally in New Zealand, it is still unusual.
"We believe it is the first time sisters have raced against each other with female co-drivers."
The Lawrence Rally is run by the Otago Sports Car Club in conjunction with the local community and businesses in Lawrence.
This year's rally will be contested over five stages and totals 77.24km.
It gets under way on Saturday morning.
