He spotted a fishing rod on its own bent over further along the beach.
When he walked toward it he saw a man in trouble in the surf, yelling for help.
The sea conditions, which had been relatively calm, got increasingly worse as the tide changed and the wind got up, he said.
"I thought 'he is in big trouble' - the waves were charging in on him and I got pretty concerned and rang 111."
Mr Burgess then saw Ms Webster swim out to help.
"She couldn't quite reach him because of the rough sea and they started to both get into trouble."
A Pukehina Volunteer Fire Brigade rescuer with a buoyancy belt swam to the man but was unable to reach Ms Webster, he said.
Volunteer firemen on shore attempted to throw a rope to Ms Webster as she swam on her back towards the beach.
"We just couldn't get the rope close enough to her," Mr Burgess said.
A jet skier was thrown off his jet ski and separated from it when he tried to rescue the couple, Mr Burgess said.
Ms Webster was eventually brought back to shore unconscious, he said.
"The rescue people worked on her for around 20-25 minutes but were not able to save her."
Her partner was helped back to shore on the retrieved jet ski.
"It was a really tragic thing to happen," said Mr Burgess. "Only a few hours beforehand the conditions were quite safe and the couple had been happily swimming [at] the beach."
Pukehina volunteer fire brigade senior firefighter Ivan Burgener said it was immediately obvious how dire things were when they arrived after someone raised the alarm.
"When we got there we saw two people out there and one of our swimmers went out with a life tube. He went to the guy first because he was the only one moving around," he said.
The man had taken on some water and was "certainly on limited time".
"He was getting near to drowning himself really. He had to be assisted out of the water and he wasn't walking around afterward. He was pretty well buggered."
Mr Burgener said the man sat next to Ms Webster watching in silence as they worked to revive her. All of the rescuers had taken on water themselves.
"It was a huge, huge, huge thing - that whole incident was a very big incident," Mr Burgener said.
Ms Webster's death has been referred to the coroner. - Additional reporting Kiri Gillespie.