Here is a PHP validation class to validate Australian Business Numbers (ABN) and Australian Company Numbers (ACN)

abn_validator.php

<?php

/**
 * ABN and ACN Validator Class
 * @author Paul Ferrett, 2009 (http://www.paulferrett.com)
 */
class AbnValidator {

  /**
   * Return true if $number is a valid ABN
   * @param string $number
   * @return bool True if $number is a valid ABN
   */
  public static function isValidAbnOrAcn($number) {
    $number = preg_replace('/[^0-9]/', '', $number);
    if(strlen($number) == 9) {
      return self::isValidAcn($number);
    }
    if(strlen($number) == 11) {
      return self::isValidAbn($number);
    }
    return false;
  }


  /**
   * Validate an Australian Business Number (ABN)
   * @param string $abn
   * @link http://www.ato.gov.au/businesses/content.asp?doc=/content/13187.htm
   * @return bool True if $abn is a valid ABN, false otherwise
   */
  public static function isValidAbn($abn) {
    $weights = array(10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19);

    // Strip non-numbers from the acn
    $abn = preg_replace('/[^0-9]/', '', $abn);

    // Check abn is 11 chars long
    if(strlen($abn) != 11) {
      return false;
    }

    // Subtract one from first digit
    $abn[0] = ((int)$abn[0] - 1);

    // Sum the products
    $sum = 0;
    foreach(str_split($abn) as $key => $digit) {
      $sum += ($digit * $weights[$key]);
    }

    if(($sum % 89) != 0) {
      return false;
    }

    return true;
  }


  /**
   * Validate an Australian Company Number (ACN)
   * @param string $acn
   * @link http://www.asic.gov.au/asic/asic.nsf/byheadline/Australian+Company+Number+(ACN)+Check+Digit
   * @return bool True if $acn is a valid ACN, false otherwise
   */
  public static function isValidAcn($acn){
    $weights = array(8, 7, 6, 5, 4, 3, 2, 1, 0);

    // Strip non-numbers from the acn
    $acn = preg_replace('/[^0-9]/', '', $acn);

    // Check acn is 9 chars long
    if(strlen($acn) != 9) {
      return false;
    }

    // Sum the products
    $sum = 0;
    foreach(str_split($acn) as $key => $digit) {
      $sum += $digit * $weights[$key];
    }

    // Get the remainder
    $remainder = $sum % 10;

    // Get remainder compliment
    $complement = (string)(10 - $remainder);

    // If complement is 10, set to 0
    if($complement === "10") {
      $complement = "0";
    }

    return ($acn[8] === $complement);
  }
}

// Example Usage:
// AbnValidator::isValidAbnOrAcn("44 706 210 937")
//  => true

The ABN format guide has since been moved to https://abr.business.gov.au/Help/AbnFormat

Super helpful. Thanks.