You can achieve it using RegEx as well - 

    $anchorTagPattern = '(?is)<a[^>]*>(.*?)<\/a>';
    $hrefPattern = '(?is)href=\"[^>]*(.*?)\"';
    #You can get Html content from item field
    #$htmlContent = $item.Fields["Your Rich Text Content Field"].Value;
    $htmlContent = @"
    <div class="test">
       <div class="test">
        <blockquote>hello<br />
            <span class="test">hello</span>
        </blockquote>
        <a href="test.com">test</a>
      </div>
    </div>
    "@;
    $anchorTagPatternMatches = [regex]::Matches($htmlContent, $anchorTagPattern);

    if(![string]::IsNullOrEmpty($anchorTagPatternMatches)) {
        write-host $anchorTagPatternMatches
        $anchorText = $anchorTagPatternMatches[0].Groups[1].Value;
        $anchorTag = $anchorTagPatternMatches[0].Value;
        write-host $anchorText
        if(![string]::IsNullOrEmpty($anchorTag)){   
            $hrefPatternMatches = [regex]::Matches($anchorTag, $hrefPattern);
            $href = $hrefPatternMatches[0].Value;
            $href = $href.Replace('href=','');
            write-host $href
            }
    }