"After this announcement you will all be permitted to move to the other side of the road, to begin this new era in our history," the New Zealand Herald quoted him as saying.
Emergency vehicles and government workers were reportedly stationed at every junction and corner but witnesses said the change happened smoothly.
"All we want to see is how smooth it is and how safe it is and I think we've seen that this morning. You see how cars are moving now, so to me it's good," a resident in the capital Apia told Reuters news .
However, the switch was briefly disrupted by a protest in a village on Savai'i - the bigger of the nation's two main islands.
The protesters blocked roads for several hours, but later allowed traffic to pass through.
The changeover survived a late legal appeal by the protest group People Against Switching Sides (Pass), who had argued that it would bring mayhem to the highways and byways of this remote South Pacific nation.
Bus drivers have also protested that their doors will now open on the wrong side, in the middle of the road.
The Samoan government introduced the change to end its reliance on expensive, left-hand drive imports from America.
It hopes that Samoan expatriates in Australia and New Zealand will now ship used, more affordable vehicles back to their homeland.
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