"He knew where the safe was and pushed her to it," he said.
"I don't know if he was armed but he was quite a big guy. He towered over her and pushed her about."
The offender tied up the woman's hands and locked her in a toilet. Mr Swenson said he did not know how she had got herself out. She was shaken but not injured.
Police said it was not clear if the man left on foot or had a vehicle. The offender wore a dark top with a distinctive yellow fluorescent design on the back, dark trousers, and black and white shoes or trainers.
The most recent police statistics showed there were five robberies in the Rotorua area in June this year, compared to three in 2014. Last month, two Rotorua dairies were robbed - with offenders allegedly using a gun during one and a golf club during the other.
Rotorua police area commander Inspector Bruce Horne said: "Our robbery rate is pretty low overall but clearly we've had a spate, and that's going to get people alarmed.
"Our clearance rate is very high, we've made arrests for both aggravated robberies at dairies on Sunset Rd and the public played a key role in helping us."
He said police were taking the latest offence seriously .
"The worry is where are they going to strike next, and we understand that."
He said business owners could take a number of precautions. They included acknowledging everyone who walked into their shop, organise their business so they could see people walking in, and removing money from the premises.
"Some businesses have signs up saying they don't carry cash, which are helpful. You shouldn't be holding large amounts of cash on the premises. Crime is all about opportunity and if you can remove those opportunities we will reduce crime."
