* Leighton asks Australian regulators to force ACS bid
* Says ACS should make fully priced bid for minorities
* Move could add at least $5.15 billion to ACS's bill
* Hochtief shares fall 1.5 percent
* Leighton shares down 0.3 percent, ACS down 1 percent
(Recasts, updates shares, adds Bilfinger Berger CEO comment, background)
By Sonali Paul and Josie Cox
MELBOURNE/FRANKFURT, Oct 18 (Reuters) - Spanish builder ACS came under fresh attack on Monday for its low-ball bid for Germany's Hochtief, as the latter's Australian unit Leighton sought regulatory action against the terms of the offer.
Leighton, of which Hochtief owns about 54 percent, joined its parent in asking Australian regulators to force ACS to make a fully priced cash bid for the subsidiary.
"We want to make it clear that ... Leighton is the biggest contributor to Hochtief, therefore we should be seriously part of the takeover offer," Leighton's Chief Executive-elect David Stewart told Reuters.
ACS's deliberately low-priced bid, consisting of eight of its own shares for every five Hochtief -- in which it already holds 29.9 percent -- is designed to allow the Spanish company to gradually up its stake without having to offer a premium for control.
Shares in Hochtief were trading 1.1 percent lower at 62.80 euros by 1049 GMT, underperforming Frankfurt's 0.1 percent stronger blue-chip index. ACS was down 0.5 percent at 37.69 euros in Madrid.
At the minimum price set by German regulators, the proposed deal values Hochtief at 50.97 euros per share.
ACS's primary goal under the proposed deal, gaining a majority stake of Hochtief, would cost about 735 million euros ($1 billion) under the existing terms.
If regulators agree to Leighton's request, the Spanish company's takeover bill would rise by at least $5.15 billion.
MANDATORY BID
Hochtief has enlisted Credit Suisse, Goldman Sachs and Deutsche Bank as advisers on its defence options. It has said a bid by ACS should trigger a mandatory bid for all of the minority interests in Leighton.
Leighton's Stewart said the company has had no contact with ACS and has no information about its intentions other than what has been reported in the media.
"The value equation is a concern to us," Stewart said. "If (ACS) were to take out a majority of Hochtief, they would then get control of Leighton for a very, very low premium."
Leighton's market capitalisation of about $11.2 billion exceeds Hochtief's $6.2 billion.
The prospect of ACS having to buy out Leighton is considered unlikely by regulatory experts, but it remains a worrying possibility for the Spanish company, which was not bargaining on buying out the 45.5 percent of Leighton not owned by Hochtief.
Hochtief has already explored a number of options to foil the bid. On Sunday, a German cabinet source told Reuters Germany is considering tightening its takeover laws in view of the acquisition attempt.
Last week, Chancellor Angela Merkel weighed in, saying Hochtief should remain based in Germany, after ACS asked investors to approve a 157 million share issue to show it could finance its proposed takeover.
In an interview published on Saturday, the chief executive of Hochtief's smaller peer Bilfinger Berger told Frankfurter Allgemeine Zeitung he would be prepared to buy a stake in Hochtief if a consortium emerged to fend off ACS's approach.
Due to its size, however, Bilfinger would not be able to take on a blocking minority as a white knight, he said.
Leighton's shares fell 0.3 percent A$37.32, outperforming a 0.8 percent fall in the broader Australian market. (Editing by Mark Bendeich and David Holmes) ($1=1.009 Australian Dollar) ($1=.7153 Euro)