Halfpenny kicked another nerveless three points, after the Lions earned a further penalty at the scrum.
When the Australian forwards couldn't live with the Lions at scrum time yet again, prop Ben Alexander was sent to the sin bin.
Halfpenny again made no mistake, to extend the Lions' lead to 19-3 after 26 minutes.
More top news
A very unsettled day with frequent outbreaks of persistent, occasionally heavy rain, especially in the south and over hills.
Protesters are set to march through Cardiff city centre today to campaign against local budget cuts to the arts.
Deaths from liver disease in Wales have more than doubled in the last 20 years.