Lomu, who receives kidney dialysis treatment three times a week, is expected to train with the Wellington provincial team later this week after receiving Mayhew's clearance.
His return has led to speculation the high-profile All Blacks winger could be representing Wellington as early as August 16. He hasn't played since his illness peaked in June.
Mayhew said he was surprised Lomu had so quickly surmounted his health problems. If Lomu takes the field in the National Provincial Championship later this month "people will see what a healthy-looking individual he is," Mayhew said.
"When it was first proposed that Jonah might play for Wellington I thought his renal physicians would say there 'no way'," Mayhew added. "That's not the case. Their opinion is that as long as he follows the proper protocols, there is no danger."
Mayhew said Lomu would be able to manage his dialysis treatments at Auckland Hospital around his playing schedule. He was expected to eventually begin home dialysis which would allow even greater flexibility.
Mayhew said there was "no medical reason I can think of" Lomu should not return to rugby.
"It may be he can't get up to his pre-dialysis levels of fitness," he said. "I'm hopeful he can. All I can do is give him my best advice and leave it at that.
"He's been training hard and he's fully fit. He may be a bit rusty in his rugby skills but that should be fairly easily overcome."
Wellington has not yet confirmed that Lomu is being considered for its NPC squad, which plays against Southland on August 16.