Borrows said the party now finds itself in a tough position, with leadership that is unpopular.
"[Judith Collins'] problem is that she is a polarising person. She's polarising in the electorate, but also in caucus.
"I think the caucus was right to go with her, and if they'd chosen anyone other than Judith or Gerry [Brownlee], the public would have been thinking who are these people, where have they come from."
Despite that, Borrows said it is "inevitable" that Collins will be rolled.
"The short answer is there's no doubt there will be another leader of the National Party before the next election. I don't know anyone with any credibility who doesn't think that.
"I think it's inevitable people will say, 'thanks for what you did but time to go'."
Borrows said losing the Whanganui seat to Labour was "disappointing".
"I feel pretty sad about the fact that National had such a lot of support over previous five or six elections that has now dropped off. In fact, it's gone completely the other way. I think that's really sad.
"In saying that, there is no doubt Steph [Lewis] worked really hard, and having the same candidate two elections running is good."
Borrows said the task of being Whanganui electorate MP isn't necessarily an easy one, and whoever wants the role needs to be committed to take it on.
"It's a big electorate and you have to be absolutely everywhere. You have to be 200 per cent committed.
"It's always hard to get around those sorts of things, and it's going to be difficult for Steph. She is a young mum, and there is no doubt she'll work really hard, but it's a lot about hard work but it's also about having the wind of a party on the rise."
Borrows had one piece of advice for Lewis.
"The public are going to brand you in the first 18 months. They're going to brand you as hardworking or lazy. Affable or aloof. Arrogant or humble. They're going to brand you.
"If you want to be in charge of your brand, you need to work as hard as you can. Be everywhere, go to everything, comment on everything, always be friendly, always have that smile painted on.
"If you want to be in charge of your brand, you've got to grab it with both hands."