NSScrollView scrollToTop

Finally figured it out. Works like a charm!

if ([self hasVerticalScroller]) {
    self.verticalScroller.floatValue = 0;
}

NSPoint newOrigin = NSMakePoint(0, NSMaxY(((NSView*)self.documentView).frame) - self.boundsHeight);
[self.contentView scrollToPoint:newOrigin];

Updated for Swift 5:

If your document view is flipped:

scrollView.documentView?.scroll(.zero)

otherwise:

if let documentView = scrollView.documentView {
        documentView.scroll(NSPoint(x: 0, y: documentView.bounds.size.height))
}