I am seperating lengthy posts using <!--nextpage--> but have to output user comments only around the last 'page' of the publish. I can not discover a way of counting the amount of paginated pages after which outputting if page == last page.

Any assistance is appreciated

You should check with $wp_query->query_vars['page']. On page one, this value will not is available, whereas around the subsequent pages it will likely be set using the current page number.

$currentPage = isset($wp_query->query_vars['page']) ?  (int)$wp_query->query_vars['page'] : 1;